Bài giảng MongoDB and Python

Nội dung text: Bài giảng MongoDB and Python

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3. MongoDB and Python Niall O’Higgins Beijing • Cambridge • Farnham • Kửln • Sebastopol • Tokyo www.it-ebooks.info
4. MongoDB and Python by Niall O’Higgins Copyright â 2011 Niall O’Higgins. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles ( For more information, contact our corporate/institutional sales department: (800) 998-9938 or corporate@oreilly.com. Editors: Mike Loukides and Shawn Wallace Cover Designer: Karen Montgomery Production Editor: Jasmine Perez Interior Designer: David Futato Proofreader: O’Reilly Production Services Illustrator: Robert Romano Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. MongoDB and Python, the image of a dwarf mongoose, and related trade dress are trademarks of O’Reilly Media, Inc. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and O’Reilly Media, Inc. was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and author assume no responsibility for errors or omissions, or for damages resulting from the use of the information con- tained herein. ISBN: 978-1-449-31037-0 [LSI] 1315837615 www.it-ebooks.info
5. Table of Contents Preface v 1. Getting Started 1 Introduction 1 Finding Reference Documentation 2 Installing MongoDB 3 Running MongoDB 5 Setting up a Python Environment with MongoDB 6 2. Reading and Writing to MongoDB with Python 9 Connecting to MongoDB with Python 10 Getting a Database Handle 11 Inserting a Document into a Collection 12 Write to a Collection Safely and Synchronously 13 Guaranteeing Writes to Multiple Database Nodes 14 Introduction to MongoDB Query Language 15 Reading, Counting, and Sorting Documents in a Collection 15 Updating Documents in a Collection 18 Deleting Documents from a Collection 20 MongoDB Query Operators 21 MongoDB Update Modifiers 22 3. Common MongoDB and Python Patterns 23 A Uniquely Document-Oriented Pattern: Embedding 23 Fast Lookups: Using Indexes with MongoDB 29 Location-based Apps with MongoDB: GeoSpatial Indexing 33 Code Defensively to Avoid KeyErrors and Other Bugs 37 Update-or-Insert: Upserts in MongoDB 39 Atomic Read-Write-Modify: MongoDB’s findAndModify 40 Fast Accounting Pattern 41 iii www.it-ebooks.info
6. 4. MongoDB with Web Frameworks 45 Pylons 1.x and MongoDB 45 Pyramid and MongoDB 49 Django and MongoDB 51 Going Further 53 iv | Table of Contents www.it-ebooks.info
7. Preface I’ve been building production database-driven applications for about 10 years. I’ve worked with most of the usual relational databases (MSSQL Server, MySQL, PostgreSQL) and with some very interesting nonrelational databases (Freebase.com’s Graphd/MQL, Berkeley DB, MongoDB). MongoDB is at this point the system I enjoy working with the most, and choose for most projects. It sits somewhere at a crossroads between the performance and pragmatism of a relational system and the flexibility and expressiveness of a semantic web database. It has been central to my success in building some quite complicated systems in a short period of time. I hope that after reading this book you will find MongoDB to be a pleasant database to work with, and one which doesn’t get in the way between you and the application you wish to build. Conventions Used in This Book The following typographical conventions are used in this book: Italic Indicates new terms, URLs, email addresses, filenames, and file extensions. Constant width Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords. Constant width bold Shows commands or other text that should be typed literally by the user. Constant width italic Shows text that should be replaced with user-supplied values or by values deter- mined by context. v www.it-ebooks.info
8. This icon signifies a tip, suggestion, or general note. This icon indicates a warning or caution. Using Code Examples This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “MongoDB and Python by Niall O’Higgins. Copyright 2011 O’Reilly Media Inc., 978-1-449-31037-0.” If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at permissions@oreilly.com. Safariđ Books Online Safari Books Online is an on-demand digital library that lets you easily search over 7,500 technology and creative reference books and videos to find the answers you need quickly. With a subscription, you can read any page and watch any video from our library online. Read books on your cell phone and mobile devices. Access new titles before they are available for print, and get exclusive access to manuscripts in development and post feedback for the authors. Copy and paste code samples, organize your favorites, down- load chapters, bookmark key sections, create notes, print out pages, and benefit from tons of other time-saving features. O’Reilly Media has uploaded this book to the Safari Books Online service. To have full digital access to this book and others on similar topics from O’Reilly and other pub- lishers, sign up for free at vi | Preface www.it-ebooks.info
9. How to Contact Us Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We have a web page for this book, where we list errata, examples, and any additional information. You can access this page at: To comment or ask technical questions about this book, send email to: bookquestions@oreilly.com For more information about our books, courses, conferences, and news, see our website at Find us on Facebook: Follow us on Twitter: Watch us on YouTube: Acknowledgments I would like to thank Ariel Backenroth, Aseem Mohanty and Eugene Ciurana for giving detailed feedback on the first draft of this book. I would also like to thank the O’Reilly team for making it a great pleasure to write the book. Of course, thanks to all the people at 10gen without whom MongoDB would not exist and this book would not have been possible. Preface | vii www.it-ebooks.info
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11. CHAPTER 1 Getting Started Introduction First released in 2009, MongoDB is relatively new on the database scene compared to contemporary giants like Oracle which trace their first releases to the 1970’s. As a document-oriented database generally grouped into the NoSQL category, it stands out among distributed key value stores, Amazon Dynamo clones and Google BigTable re- implementations. With a focus on rich operator support and high performance Online Transaction Processing (OLTP), MongoDB is in many ways closer to MySQL than to batch-oriented databases like HBase. The key differences between MongoDB’s document-oriented approach and a tradi- tional relational database are: 1. MongoDB does not support joins. 2. MongoDB does not support transactions. It does have some support for atomic operations, however. 3. MongoDB schemas are flexible. Not all documents in a collection must adhere to the same schema. 1 and 2 are a direct result of the huge difficulties in making these features scale across a large distributed system while maintaining acceptable performance. They are trade- offs made in order to allow for horizontal scalability. Although MongoDB lacks joins, it does introduce some alternative capabilites, e.g. embedding, which can be used to solve many of the same data modeling problems as joins. Of course, even if embedding doesn’t quite work, you can always perform your join in application code, by making multiple queries. The lack of transactions can be painful at times, but fortunately MongoDB supports a fairly decent set of atomic operations. From the basic atomic increment and decrement operators to the richer “findAndModify”, which is essentially an atomic read-modify- write operator. 1 www.it-ebooks.info
12. It turns out that a flexible schema can be very beneficial, especially when you expect to be iterating quickly. While up front schema design—as used in the relational model —has its place, there is often a heavy cost in terms of maintenance. Handling schema updates in the relational world is of course doable, but comes with a price. In MongoDB, you can add new properties at any time, dynamically, without having to worry about ALTER TABLE statements that can take hours to run and complicated data migration scripts. However, this approach does come with its own tradeoffs. For example, type enforcement must be carefully handled by the application code. Custom document versioning might be desirable to avoid large conditional blocks to handle heterogeneous documents in the same collection. The dynamic nature of MongoDB lends itself quite naturally to working with a dynamic language such as Python. The tradeoffs between a dynamically typed language such as Python and a statically typed language such as Java in many respects mirror the trade- offs between the flexible, document-oriented model of MongoDB and the up-front and statically typed schema definition of SQL databases. Python allows you to express MongoDB documents and queries natively, through the use of existing language features like nested dictionaries and lists. If you have worked with JSON in Python, you will immediately be comfortable with MongoDB documents and queries. For these reasons, MongoDB and Python make a powerful combination for rapid, iter- ative development of horizontally scalable backend applications. For the vast majority of modern Web and mobile applications, we believe MongoDB is likely a better fit than RDBMS technology. Finding Reference Documentation MongoDB, Python, 10gen’s PyMongo driver and each of the Web frameworks men- tioned in this book all have good reference documentation online. For MongoDB, we would strongly suggest bookmarking and at least skimming over the official MongoDB manual which is available in a few different formats and con- stantly updated at While the manual describes the JavaScript interface via the mongo console utility as opposed to the Python interface, most of the code snippets should be easily understood by a Python program- mer and more-or-less portable to PyMongo, albeit sometimes with a little bit of work. Furthermore, the MongoDB manual goes into greater depth on certain advanced and technical implementation and database administration topics than is possible in this book. 2 | Chapter 1: Getting Started www.it-ebooks.info
13. For the Python language and standard library, you can use the help() function in the interpreter or the pydoc tool on the command line to get API documentation for any methods or modules. For example: pydoc string The latest Python language and API documentation is also available for online browsing at 10gen’s PyMongo driver has API documentation available online to go with each re- lease. You can find this at Additionally, once you have the PyMongo driver package installed on your system, a summary version of the API documentation should be available to you in the Python interpreter via the help() function. Due to an issue with the virtualenv tool mentioned in the next section, “py- doc” does not work inside a virtual environment. You must instead run python -m pydoc pymongo. Installing MongoDB For the purposes of development, it is recommended to run a MongoDB server on your local machine. This will permit you to iterate quickly and try new things without fear of destroying a production database. Additionally, you will be able to develop with MongoDB even without an Internet connection. Depending on your operating system, you may have multiple options for how to install MongoDB locally. Most modern UNIX-like systems will have a version of MongoDB available in their package management system. This includes FreeBSD, Debian, Ubuntu, Fedora, Cen- tOS and ArchLinux. Installing one of these packages is likely the most convenient ap- proach, although the version of MongoDB provided by your packaging vendor may lag behind the latest release from 10gen. For local development, as long as you have the latest major release, you are probably fine. 10gen also provides their own MongoDB packages for many systems which they update very quickly on each release. These can be a little more work to get installed but ensure you are running the latest-and-greatest. After the initial setup, they are typically trivial to keep up-to-date. For a production deployment, where you likely want to be able to update to the most recent stable MongoDB version with a minimum of hassle, this option probably makes the most sense. In addition to the system package versions of MongoDB, 10gen provide binary zip and tar archives. These are independent of your system package manager and are provided in both 32-bit and 64-bit flavours for OS X, Windows, Linux and Solaris. 10gen also provide statically-built binary distributions of this kind for Linux, which may be your best option if you are stuck on an older, legacy Linux system lacking the modern libc Installing MongoDB | 3 www.it-ebooks.info
14. and other library versions. Also, if you are on OS X, Windows or Solaris, these are probably your best bet. Finally, you can always build your own binaries from the source code. Unless you need to make modifications to MongoDB internals yourself, this method is best avoided due to the time and complexity involved. In the interests of simplicity, we will provide the commands required to install a stable version of MongoDB using the system package manager of the most common UNIX- like operating systems. This is the easiest method, assuming you are on one of these platforms. For Mac OS X and Windows, we provide instructions to install the binary packages from 10gen. Ubuntu / Debian: sudo apt-get update; sudo apt-get install mongodb Fedora: sudo yum install mongo-stable-server FreeBSD: sudo pkg_add -r mongodb Windows: Go to and download the latest production release zip file for Windows—choosing 32-bit or 64-bit depending on your system. Extract the contents of the zipfile to a location like C:\mongodb and add the bin directory to your PATH. Mac OS X: Go to and download the latest production release compressed tar file for OS X—choosing 32-bit or 64-bit depending on your system. Extract the contents to a location like /usr/local/ or /opt and add the bin directory to your $PATH. For exmaple: cd /tmp wget tar xfz mongodb-osx-x86_64-1.8.3-rc1.tgz sudo mkdir /usr/local/mongodb sudo cp -r mongodb-osx-x86_64-1.8.3-rc1/bin /usr/local/mongodb/ export PATH=$PATH:/usr/local/mongodb/bin 4 | Chapter 1: Getting Started www.it-ebooks.info
15. Install MongoDB on OS X with Mac Ports If you would like to try a third-party system package management system on Mac OS X, you may also install MongoDB (and Python, in fact) through Mac Ports. Mac Ports is similar to FreeBSD ports, but for OS X. A word of warning though: Mac Ports compiles from source, and so can take consid- erably longer to install software compared with simply grabbing the binaries. Futher- more, you will need to have Apple’s Xcode Developer Tools installed, along with the X11 windowing environment. The first step is to install Mac Ports from We recommend downloading and installing their DMG package. Once you have Mac Ports installed, you can install MongoDB with the command: sudo port selfupdate; sudo port install mongodb To install Python 2.7 from Mac Ports use the command: sudo port selfupdate; sudo port install python27 Running MongoDB On some platforms—such as Ubuntu—the package manager will automatically start the mongod daemon for you, and ensure it starts on boot also. On others, such as Mac OS X, you must write your own script to start it, and manually integrate with launchd so that it starts on system boot. Note that before you can start MongoDB, its data and log directories must exist. If you wish to have MongoDB start automatically on boot on Windows, 10gen have a document describing how to set this up at Windows+Service To have MongoDB start automatically on boot under Mac OS X, first you will need a plist file. Save the following (changing db and log paths appropriately) to /Library/ LaunchDaemons/org.mongodb.mongod.plist: RunAtLoad Label org.mongo.mongod ProgramArguments /usr/local/mongodb/bin/mongod dbpath Running MongoDB | 5 www.it-ebooks.info
16. /usr/local/mongodb/data/ logpath /usr/local/mongodb/log/mongodb.log Next run the following commands to activate the startup script with launchd: sudo launchctl load /Library/LaunchDaemons/org.mongodb.mongod.plist sudo launchctl start org.mongodb.mongod A quick way to test whether there is a MongoDB instance already running on your local machine is to type mongo at the command-line. This will start the MongoDB admin console, which attempts to connect to a database server running on the default port (27017). In any case, you can always start MongoDB manually from the command-line. This is a useful thing to be familiar with in case you ever want to test features such as replica sets or sharding by running multiple mongod instances on your local machine. Assuming the mongod binary is in your $PATH, run: mongod logpath port dbpath Setting up a Python Environment with MongoDB In order to be able to connect to MongoDB with Python, you need to install the Py- Mongo driver package. In Python, the best practice is to create what is known as a “virtual environment” in which to install your packages. This isolates them cleanly from any “system” packages you have installed and yields the added bonus of not requiring root privileges to install additional Python packages. The tool to create a “virtual environment” is called virtualenv. There are two approaches to installing the virtualenv tool on your system—manually and via your system package management tool. Most modern UNIX-like systems will have the virtualenv tool in their package repositories. For example, on Mac OS X with Mac Ports, you can run sudo port install py27-virtualenv to install virtualenv for Python 2.7. On Ubuntu you can run sudo apt-get install python-virtualenv. Refer to the documentation for your OS to learn how to install it on your specific platform. In case you are unable or simply don’t want to use your system’s package manager, you can always install it yourself, by hand. In order to manually install it, you must have the Python setuptools package. You may already have setuptools on your system. You can test this by running python -c import setuptools on the command line. If nothing is printed and you are simply returned to the prompt, you don’t need to do anything. If an ImportError is raised, you need to install setuptools. 6 | Chapter 1: Getting Started www.it-ebooks.info
17. To manually install setuptools, first download the file dist/ez_setup.py Then run python ez_setup.py as root. For Windows, first download and install the latest Python 2.7.x package from http:// www.python.org. Once you have installed Python, download and install the Windows setuptools installer package from After instal- ling Python 2.7 and setuptools, you will have the easy_install tool available on your machine in the Python scripts directory—default is C:\Python27\Scripts\. Once you have setuptools installed on your system, run easy_install virtualenv as root. Now that you have the “virtualenv” tool available on your machine, you can create your first virtual Python environment. You can do this by executing the command virtualenv no-site-packages myenv. You do not need—and indeed should not want —to run this command with root privileges. This will create a virtual environment in the directory “myenv”. The no-site-packages option to the “virtualenv” utility in- structs it to create a clean Python environment, isolated from any existing packages installed in the system. You are now ready to install the PyMongo driver. With the “myenv” directory as your working directory (i.e. after “cd myenv”), simply execute bin/easy_install pymongo. This will install the latest stable version of PyMongo into your virtual Python environment. To verify that this worked successfully, execute the command bin/python -c import pymongo, making sure that the “myenv” directory is still your working directory, as with the previous command. Assuming Python did not raise an ImportError, you now have a Python virtualenv with the PyMongo driver correctly installed and are ready to connect to MongoDB and start issuing queries! Setting up a Python Environment with MongoDB | 7 www.it-ebooks.info
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19. CHAPTER 2 Reading and Writing to MongoDB with Python MongoDB is a document-oriented database. This is different from a relational database in two significant ways. Firstly, not all entries must adhere to the same schema. Sec- ondly you can embed entries inside of one another. Despite these major differences, there are analogs to SQL concepts in MongoDB. A logical group of entries in a SQL database is termed a table. In MongoDB, the analogous term is a collection. A single entry in a SQL databse is termed a row. In MongoDB, the analog is a document. Table 2-1. Comparison of SQL/RDBMS and MongoDB Concepts and Terms Concept SQL MongoDB One User One Row One Document All Users Users Table Users Collection One Username Per User (1-to-1) Username Column Username Property Many Emails Per User (1-to-many) SQL JOIN with Emails Table Embed relevant email doc in User Document Many Items Owned by Many Users (many-to- SQL JOIN with Items Table Programmatically Join with Items many) Collection Hence, in MongoDB, you are mostly operating on documents and collections of docu- ments. If you are familiar with JSON, a MongoDB document is essentially a JSON document with a few extra features. From a Python perspective, it is a Python diction- ary. Consider the following example of a user document with a username, first name, sur- name, date of birth, email address and score: from datetime import datetime user_doc = { "username" : "janedoe", "firstname" : "Jane", 9 www.it-ebooks.info
20. "surname" : "Doe", "dateofbirth" : datetime(1974, 4, 12), "email" : "janedoe74@example.com", "score" : 0 } As you can see, this is a native Python object. Unlike SQL, there is no special syntax to deal with. The PyMongo driver transparently supports Python datetime objects. This is very convenient when working with datetime instances—the driver will transparently marshall the values for you in both reads and writes. You should never have to write datetime conversion code yourself. Instead of grouping things inside of tables, as in SQL, MongoDB groups them in col- lections. Like SQL tables, MongoDB collections can have indexes on particular docu- ment properties for faster lookups and you can read and write to them using complex query predicates. Unlike SQL tables, documents in a MongoDB collection do not all have to conform to the same schema. Returning to our user example above, such documents would be logically grouped in a “users” collection. Connecting to MongoDB with Python The PyMongo driver makes connecting to a MongoDB database quite straight forward. Furthermore, the driver supports some nice features right out of the box, such as con- nection pooling and automatic reconnect on failure (when working with a replicated setup). If you are familiar with more traditional RDBMS/SQL systems—for example MySQL—you are likely used to having to deploy additional software, or possibly even write your own, to handle connection pooling and automatic reconnect. 10gen very thoughtfully relieved us of the need to worry about these details when working with MongoDB and the PyMongo driver. This takes a lot of the headache out of running a production MongoDB-based system. You instantiate a Connection object with the necessary parameters. By default, the Connection object will connect to a MongoDB server on localhost at port 27017. To be explicit, we’ll pass those parameters along in our example: """ An example of how to connect to MongoDB """ import sys from pymongo import Connection from pymongo.errors import ConnectionFailure def main(): """ Connect to MongoDB """ try: c = Connection(host="localhost", port=27017) print "Connected successfully" except ConnectionFailure, e: sys.stderr.write("Could not connect to MongoDB: %s" % e) 10 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
21. sys.exit(1) if __name__ == "__main__": main() As you can see, a ConnectionFailure exception can be thrown by Connection instan- tiation. It is usually a good idea to handle this exception and output something infor- mative to your users. Getting a Database Handle Connection objects themselves are not all that frequently used when working with MongoDB in Python. Typically you create one once, and then forget about it. This is because most of the real interaction happens with Database and Collection objects. Connection objects are just a way to get a handle on your first Databse object. In fact, even if you lose reference to the Connection object, you can always get it back because Database objects have a reference to the Connection object. Getting a Database object is easy once you have a Connection instance. You simply need to know the name of the database, and the username and password to access it if you are using authorization on it. """ An example of how to get a Python handle to a MongoDB database """ import sys from pymongo import Connection from pymongo.errors import ConnectionFailure def main(): """ Connect to MongoDB """ try: c = Connection(host="localhost", port=27017) except ConnectionFailure, e: sys.stderr.write("Could not connect to MongoDB: %s" % e) sys.exit(1) # Get a Database handle to a database named "mydb" dbh = c["mydb"] # Demonstrate the db.connection property to retrieve a reference to the # Connection object should it go out of scope. In most cases, keeping a # reference to the Database object for the lifetime of your program should # be sufficient. assert dbh.connection == c print "Successfully set up a database handle" if __name__ == "__main__": main() Getting a Database Handle | 11 www.it-ebooks.info
22. Inserting a Document into a Collection Once you have a handle to your database, you can begin inserting data. Let us imagine we have a collection called “users”, containing all the users of our game. Each user has a username, a first name, surname, date of birth, email address and a score. We want to add a new user: """ An example of how to insert a document """ import sys from datetime import datetime from pymongo import Connection from pymongo.errors import ConnectionFailure def main(): try: c = Connection(host="localhost", port=27017) except ConnectionFailure, e: sys.stderr.write("Could not connect to MongoDB: %s" % e) sys.exit(1) dbh = c["mydb"] assert dbh.connection == c user_doc = { "username" : "janedoe", "firstname" : "Jane", "surname" : "Doe", "dateofbirth" : datetime(1974, 4, 12), "email" : "janedoe74@example.com", "score" : 0 } dbh.users.insert(user_doc, safe=True) print "Successfully inserted document: %s" % user_doc if __name__ == "__main__": main() Note that we don’t have to tell MongoDB to create our collection “users” before we insert to it. Collections are created lazily in MongoDB, whenever you access them. This has the advantage of being very lightweight, but can occasionally cause problems due to typos. These can be hard to track down unless you have good test coverage. For example, imagine you accidentally typed: # dbh.usrs is a typo, we mean dbh.users! Unlike an RDBMS, MongoDB won't # protect you from this class of mistake. dbh.usrs.insert(user_doc) The code would execute correctly and no errors would be thrown. You might be left scratching your head wondering why your user document isn’t there. We recommend being extra vigilant to double check your spelling when addressing collections. Good test coverage can also help find bugs of this sort. 12 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
23. Another feature of MongoDB inserts to be aware of is primary key auto-generation. In MongoDB, the _id property on a document is treated specially. It is considered to be the primary key for that document, and is expected to be unique unless the collection has been explcitly created without an index on _id. By default, if no _id property is present in a document you insert, MongoDB will generate one itself. When MongoDB generates an _id property itself, it uses the type ObjectId. A MongoDB ObjectId is a 96-bit value which is expected to have a very high probability of being unique when created. It can be considered similar in purpose to a UUID object as defined by RFC 4122. MongoDB ObjectIds have the nice property of being almost-certainly-unique upon generation, hence no central coordination is required. This contrasts sharply with the common RDBMS idiom of using auto-increment pri- mary keys. Guaranteeing that an auto-increment key is not already in use usually re- quires consulting some centralized system. When the intention is to provide a hori- zontally scalable, de-centralized and fault-tolerant database—as is the case with Mon- goDB—auto-increment keys represent an ugly bottleneck. By employing ObjectId as your _id, you leave the door open to horizontal scaling via MongoDB’s sharding capabilities. While you can in fact supply your own value for the _id property if you wish—so long as it is globally unique—this is best avoided unless there is a strong reason to do otherwise. Examples of cases where you may be forced to provide your own _id property value include migration from RDBMS systems which utilized the previously-mentioned auto-increment primary key idiom. Note that an ObjectId can be just as easily generated on the client-side, with PyMongo, as by the server. To generate an ObjectId with PyMongo, you simply instantiate pymongo.objectid.ObjectId. Write to a Collection Safely and Synchronously By default, the PyMongo driver performs asynchronous writes. Write operations in- clude insert, update, remove and findAndModify. Asynchronous writes are unsafe in the sense that they are not checked for errors and so execution of your program could continue without any guarantees of the write hav- ing completed successfully. While asynchronous writes improve performance by not blocking execution, they can easily lead to nasty race conditions and other nefarious data integrity bugs. For this reason, we recommend you almost always use safe, syn- chronous, blocking writes. It seems rare in practice to have truly “fire-and-forget” writes where there are aboslutely no consequences for failures. That being said, one common example where asynchronous writes may make sense is when you are writing non- critical logs or analytics data to MongoDB from your application. Write to a Collection Safely and Synchronously | 13 www.it-ebooks.info
24. Unless you are certain you don’t need synchronous writes, we recom- mend that you pass the “safe=True” keyword argument to inserts, up- dates, removes and findAndModify operations: # safe=True ensures that your write # will succeed or an exception will be thrown dbh.users.insert(user_doc, safe=True) Guaranteeing Writes to Multiple Database Nodes The term node refers to a single instance of the MongoDB daemon process. Typically there is a single MongoDB node per machine, but for testing or development cases you can run multiple nodes on one machine. Replica Set is the MongoDB term for the database’s enhanced master-slave replication configuration. This is similar to the traditional master-slave replication you find in RDBMS such as MySQL and PostgreSQL in that a single node handles writes at a given time. In MongoDB master selection is determined by an election protocol and during failover a slave is automatically promoted to master without requiring operator inter- vention. Furthermore, the PyMongo driver is Replica Set-aware and performs auto- matic reconnect on failure to the new master. MongoDB Replica Sets, therefore, rep- resent a master-slave replication configuration with excellent failure handling out of the box. For anyone who has had to manually recover from a MySQL master failure in a production environment, this feature is a welcome relief. By default, MongoDB will return success for your write operation once it has been written to a single node in a Replica Set. However, for added safety in case of failure, you may wish your write to be committed to two or more replicas before returning success. This can help ensure that in case of catastrophic failure, at least one of the nodes in the Replica Set will have your write. PyMongo makes it easy to specify how many nodes you would like your write to be replicated to before returning success. You simply set a parameter named “w” to the number of servers in each write method call. For example: # w=2 means the write will not succeed until it has # been written to at least 2 servers in a replica set. dbh.users.insert(user_doc, w=2) Note that passing any value of “w” to a write method in PyMongo im- plies setting “safe=True” also. 14 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
25. Introduction to MongoDB Query Language MongoDB queries are represented as a JSON-like structure, just like documents. To build a query, you specify a document with properties you wish the results to match. MongoDB treats each property as having an implicit boolean AND. It natively supports boolean OR queries, but you must use a special operator ($or) to achieve it. In addition to exact matches, MongoDB has operators for greater than, less than, etc. Sample query document to match all documents in the users collection with firstname “jane”: q = { "firstname" : "jane" } If we wanted to retrieve all documents with firstname “jane” AND surname “doe”, we would write: q = { "firstname" : "jane", "surname" : "doe" } If we wanted to retrieve all documents with a score value of greater than 0, we would write: q = { "score" : { "$gt" : 0 } } Notice the use of the special “$gt” operator. The MongoDB query language provides a number of such operators, enabling you to build quite complex queries. See the section on MongoDB Query Operators for details. Reading, Counting, and Sorting Documents in a Collection In many situations, you only want to retrieve a single document from a collection. This is especially true when documents in your collection are unique on some property. A good example of this is a users collection, where each username is guaranteed unique. # Assuming we already have a database handle in scope named dbh # find a single document with the username "janedoe". user_doc = dbh.users.find_one({"username" : "janedoe"}) if not user_doc: print "no document found for username janedoe" Notice that find_one() will return None if no document is found. Now imagine you wish to find all documents in the users collection which have a firstname property set to “jane” and print out their email addresses. MongoDB will return a Cursor object for us, to stream the results. PyMongo handles result streaming Reading, Counting, and Sorting Documents in a Collection | 15 www.it-ebooks.info
26. as you iterate, so if you have a huge number of results they are not all stored in memory at once. # Assuming we already have a database handle in scope named dbh # find all documents with the firstname "jane". # Then iterate through them and print out the email address. users = dbh.users.find({"firstname":"jane"}) for user in users: print user.get("email") Notice in the above example that we use the Python dict class’ get method. If we were certain that every single result document contained the “email” property, we could have used dictionary access instead. for user in users: print user["email"] If you only wish to retrieve a subset of the properties from each document in a collection during a read, you can pass those as a dictionary via an additional parameter. For example, suppose that you only wish to retrieve the email address for each user with firstname “jane”: # Only retrieve the "email" field from each matching document. users = dbh.users.find({"firstname":"jane"}, {"email":1}) for user in users: print user.get("email") If you are retrieving a large result set, requesting only the properties you need can reduce network and decoding overhead, potentially increasing performance. Sometimes you are not so interested in the query results themselves, but are looking to find the size of the result set for a given query. A common example is an analytics situation where you want a count of how many documents are in your users’ collections. MonogDB supports efficient server-side counting of result sets with the count() method on Cursor objects: # Find out how many documents are in users collection, efficiently userscount = dbh.users.find().count() print "There are %d documents in users collection" % userscount MongoDB can also perform result sorting for you on the server-side. Especially if you are sorting results on a property which has an index, it can sort these far more efficiently than your client program can. PyMongo Cursor objects have a sort() method which takes a Python 2-tuple comprising the property to sort on, and the direction. The Py- Mongo sort() method is analogous to the SQL ORDER BY statement. Direction can either be pymongo.ASCENDING or pymongo.DESCENDING. For example: # Return all user with firstname "jane" sorted # in descending order by birthdate (ie youngest first) users = dbh.users.find( {"firstname":"jane"}).sort(("dateofbirth", pymongo.DESCENDING)) for user in users: print user.get("email") 16 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
27. In addition to the sort() method on the PyMongo Cursor object, you may also pass sort instructions to the find() and find_one() methods on the PyMongo Collection object. Using this facility, the above example may be rewritten as: # Return all user with firstname "jane" sorted # in descending order by birthdate (ie youngest first) users = dbh.users.find({"firstname":"jane"}, sort=[("dateofbirth", pymongo.DESCENDING)]) for user in users: print user.get("email") Another situation you may encounter—especially when you have large result sets—is that you wish to only fetch a limited number of results. This is frequently combined with server-side sorting of results. For example, imagine you are generating a high score table which displays only the top ten scores. PyMongo Cursor objects have a limit() method which enables this. The limit() method is analogous to the SQL LIMIT state- ment. # Return at most 10 users sorted by score in descending order # This may be used as a "top 10 users highscore table" users = dbh.users.find().sort(("score", pymongo.DESCENDING)).limit(10) for user in users: print user.get("username"), user.get("score", 0) If you know in advance that you only need a limited number of results from a query, using limit() can yield a performance benefit. This is because it may greatly reduce the size of the results data which must be sent by MongoDB. Note that a limit of 0 is equivalent to no limit. Additionally, MongoDB can support skipping to a specific offset in a result set through the Cursor.skip() method provided by PyMongo. When used with limit() this enables result pagination which is frequently used by clients when allowing end-users to browse very large result sets. skip() is analogous to the SQL OFFSET statement. For example, imagine a Web application which displays 20 users per page, sorted alphabetically by surname , and needs to fetch the data to build the second page of results for a user. The query used by the Web application might look like this: # Return at most 20 users sorted by name, # skipping the first 20 results in the set users = dbh.users.find().sort( ("surname", pymongo.ASCENDING)).limit(20).skip(20) Finally, when traversing very large result sets, where the underlying documents may be modified by other programs at the same time, you may wish to use MongoDB’s Snap- shot Mode. Imagine a busy site with hundreds of thousands of users. You are devel- oping an analytics program to count users and build various statistics about usage patterns and so on. However, this analytics program is intended to run against the live, production database. Since this is such a busy site, real users are frequently performing actions on the site which may result in modifications to their corresponding user docu- ments—while your analytics program is running. Due to quirks in MongoDB’s cur- Reading, Counting, and Sorting Documents in a Collection | 17 www.it-ebooks.info
28. soring mechanism, in this kind of situation your program could easily see duplicates in your query result set. Duplicate data could throw off the accuracy of your analysis program, and so it is best avoided. This is where Snapshot Mode comes in. MongoDB’s Snapshot Mode guarantees that documents which are modified during the lifetime of a query are returned only once in a result set. In other words, duplicates are eliminated, and you should not have to worry about them. However, Snapshot Mode does have some limitations. Snapshot Mode cannot be used with sorting, nor can it be used with an index on any property other than _id. To use Snapshot Mode with PyMongo, simply pass “snapshot=True” as a parameter to the find() method: # Traverse the entire users collection, employing Snapshot Mode # to eliminate potential duplicate results. for user in dbh.users.find(snapshot=True): print user.get("username"), user.get("score", 0) Updating Documents in a Collection Update queries in MongoDB consist of two parts: a document spec which informs the database of the set of documents to be updated, and the update document itself. The first part, the document spec, is the same as the query document which you use with find() or find_one(). The second part, the update document, can be used in two ways. The simplest is to supply the full document which will replace the matched document in the collection. For example, suppose you had the following document in your users collection: user_doc = { "username" : "janedoe", "firstname" : "Jane", "surname" : "Doe", "dateofbirth" : datetime(1974, 4, 12), "email" : "janedoe74@example.com", "score" : 0 } Now let’s say you wish to update the document with username “janedoe” to change the email address to “janedoe74@example2.com”. We build a completely new docu- ment which is identical to the original, except for the new email address. 18 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
29. # first query to get a copy of the current document import copy old_user_doc = dbh.users.find_one({"username":"janedoe"}) new_user_doc = copy.deepcopy(old_user_doc) # modify the copy to change the email address new_user_doc["email"] = "janedoe74@example2.com" # run the update query # replace the matched document with the contents of new_user_doc dbh.users.update({"username":"janedoe"}, new_user_doc, safe=True) Building the whole replacement document can be cumbersome, and worse, can intro- duce race conditions. Imagine you want to increment the score property of the “jane- doe” user. In order to achieve this with the replacement approach, you would have to first fetch the document, modify it with the incremented score, then write it back to the database. With that approach, you could easily lose other score changes if some- thing else were to update the score in between you reading and writing it. In order to solve this problem, the update document supports an additional set of MongoDB operators called “update modifiers”. These update modifiers include oper- ators such as atomic increment/decrement, atomic list push/pop and so on. It is very helpful to be aware of which update modifiers are available and what they can do when designing your application. Many of these will be described in their own recipes throughout this book. To illustrate usage of “update modifiers”, let’s return to our original example of chang- ing only the email address of the document with username “janedoe”. We can use the $set update modifier in our update document to avoid having to query before updating. $set changes the value of an individual property or a group of properties to whatever you specify. # run the update query, using the $set update modifier. # we do not need to know the current contents of the document # with this approach, and so avoid an initial query and # potential race condition. dbh.users.update({"username":"janedoe"}, {"$set":{"email":"janedoe74@example2.com"}}, safe=True) You can also set multiple properties at once using the $set update modifier: # update the email address and the score at the same time # using $set in a single write. dbh.users.update({"username":"janedoe"}, {"$set":{"email":"janedoe74@example2.com", "score":1}}, safe=True) At the time of writing, the PyMongo driver, even if you specify a docu- ment spec to the update method which matches multiple documents in a collection, only applies the update to the first document matched. Updating Documents in a Collection | 19 www.it-ebooks.info
30. In other words, even if you believe your update document spec matches every single document in the collection, your update will only write to one of those documents. For example, let us imagine we wish to set a flag on every document in our users col- lection which has a score of 0: # even if every document in your collection has a score of 0, # only the first matched document will have its "flagged" property set to True. dbh.users.update({"score":0},{"$set":{"flagged":True}}, safe=True) In order to have your update query write multiple documents, you must pass the “multi=True” parameter to the update method. # once we supply the "multi=True" parameter, all matched documents # will be updated dbh.users.update({"score":0},{"$set":{"flagged":True}}, multi=True, safe=True) Although the default value for the multi parameter to the update method is currently False—meaning only the first matched document will be updated—this may change. The PyMongo documentation currently recommends that you explicitly set multi=False if you are relying on this default, to avoid breakage in future. Note that this should only impact you if you are working with a collection where your documents are not unique on the property you are querying on in your document spec. Deleting Documents from a Collection If you wish to permanently delete documents from a collection, it is quite easy to do so. The PyMongo Collection object has a remove() method. As with reads and updates, you specify which documents you want to remove by way of a document spec. For example, to delete all documents from the users collection with a score of 1, you would use the following code: # Delete all documents in user collection with score 1 dbh.users.remove({"score":1}, safe=True) Note that the remove() method takes a safe parameter. As mentioned in the earlier section “Write to a Collection Safely and Synchronously”, it is recommended to set the safe parameter to True on write methods to ensure the operation has completed. It is also worth noting that remove() will not raise any exception or error if no documents are matched. 20 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
31. Finally, if you wish to delete all documents in a collection, you can pass None as a parameter to remove(): # Delete all documents in user collection dbh.users.remove(None, safe=True) Clearing a collection with remove() differs from dropping the collection via drop_col lection() in that the indexes will remain intact. MongoDB Query Operators As mentioned previously, MongoDB has quite a rich set of query operators and pred- icates. In Table 2-2 we provide a table with the meaning of each one, along with a sample usage and the SQL equivalent where applicable. Table 2-2. MongoDB query operators Operator Meaning Example SQL Equivalent $gt Greater Than “score”:{"$gt”:0} > $lt Less Than “score”:{"$lt”:0} = $lte Less Than or Equal “score”:{"$lte”:0} ⇐ $all Array Must Contain All “skills”:{"$all”:["mongodb”,"python"]} N/A $exists Property Must Exist “email”:{"$exists”:True} N/A $mod Modulo X Equals Y “seconds”:{"$mod”:[60,0]} MOD() $ne Not Equals “seconds”:{"$ne”:60} != $in In “skills”:{"$in”:["c”,"c++"]} IN $nin Not In “skills”:{"$nin”:["php”,"ruby”,"perl"]} NOT IN $nor Nor “$nor”:[{"language”:"english"},{"coun- N/A try”:"usa"}] $or Or “$or”:[{"language”:"english"},{"coun- OR try”:"usa"}] $size Array Must Be Of Size “skills”:{"$size”:3} N/A If you do not fully understand the meaning or purpose of some of these operators immediately do not worry. We shall discuss the practical use of some of the more advanced operators in detail in Chapter 3. MongoDB Query Operators | 21 www.it-ebooks.info
32. MongoDB Update Modifiers As mentioned in the section “Updating Documents in a Collection”, MongoDB comes with a set of operators for performing atomic modifications on documents. Table 2-3. MongoDB update modifiers Modifier Meaning Example $inc Atomic Increment “$inc”:{"score”:1} $set Set Property Value “$set”:{"username”:"niall"} $unset Unset (delete) Property “$unset”:{"username”:1} $push Atomic Array Append (atom) “$push”:{"emails”:"foo@example.com"} $pushAll Atomic Array Append (list) “$pushall”:{"emails”:["foo@example.com”,"foo2@ex- ample.com"]} $addToSet Atomic Append-If-Not-Present “$addToSet”:{"emails”:"foo@example.com"} $pop Atomic Array Tail Remove “$pop”:{"emails”:1} $pull Atomic Conditional Array Item “$pull”:{"emails”:"foo@example.com"} Removal $pullAll Atomic Array Multi Item Re- “$pullAll”:{"emails”:["foo@example.com”, “foo2@ex- moval ample.com"]} $rename Atomic Property Rename “$rename”:{"emails”:"old_emails"} As with the MongoDB query operators listed earlier in this chapter, this table is mostly for your reference. These operators will be introduced in greater detail in Chapter 3. 22 | Chapter 2: Reading and Writing to MongoDB with Python www.it-ebooks.info
33. CHAPTER 3 Common MongoDB and Python Patterns After some time working with MongoDB and Python to solve different problems, var- ious patterns and best practices begin to emerge. Just as with any programming lan- guage and database system, there are established approaches for modeling data along with known methods for answering queries as quickly and efficiently as possible. While there are myriad sources of such knowledge for traditional RDBM systems like MySQL, there are far fewer resources available for MongoDB. This chapter is an at- tempt to address this. A Uniquely Document-Oriented Pattern: Embedding While the ability of MongoDB documents to contain sub-documents has been men- tioned previously in this book, it has not been explored in detail. In fact, embedding is an extremely important modeling technique when working with MongoDB and can have important performance and scalability implications. In particular, embedding can be used to solve many data modeling problems usually solved by a join in traditional RDBMS. Furthermore, embedding is perhaps more intuitive and easier to understand than a join. What exactly is meant by embedding? In Python terms, when the value of a key in a dictionary is yet another dictionary, we say that you are embedding the latter in the former. For example: my_document = { "name":"foo document", "data":{"name":"bar document"} } Here, “data” is a sub-document embedded in the top-level document “my_document”. 23 www.it-ebooks.info
34. Embedding sub-documents can be a useful, natural technique to reduce clutter or namespace collisions. For example consider the case where a “user” document should reference Facebook, Twitter and IRC account usernames, passwords and associated details—in addition to storing a “username” property native to your applicaton: user_doc = { "username":"foouser", "twitter":{ "username":"footwitter", "password":"secret", "email":"twitter@example.com" }, "facebook":{ "username":"foofacebook", "password":"secret", "email":"facebook@example.com" }, "irc":{ "username":"fooirc", "password":"secret", } } Note that in MongoDB documents—just as in Python dictionaries— property names (a.k.a. keys) are unique. In other words, a single docu- ment can only ever have one “username” property. This rule also applies to properties in embedded sub-documents. This uniqueness constraint can actually be exploited and enable some useful patterns. Specifically, see the section titled “Fast Accounting Pattern”. Of course, embedded sub-documents can be queried against just like their top-level counterparts. For example, it would be completely legal to attempt to query for the above document in a collection called “users” with the following statement: user_doc = dbh.users.find_one({"facebook.username":"foofacebook"}) As you can see, the dot (“.”) is used to denote keys in an embedded sub-document. This should be familiar to anybody who has worked with objects in JavaScript, where object-style access via the dot notation can be used in parallel with dictionary-style access via square brackets. As MongoDB uses JavaScript heavily internally, this choice of notation is unsurprising. JSON is JavaScript Object Notation, afterall. The dot no- tation can also be used in update statements with update modifiers such as $set to set the value of an individual sub-property: # update modifiers such as $set also support the dot notation dbh.users.update({"facebook.username":"foofacebook"}, {"$set":{"facebook.username":"bar"}}, safe=True) This use of embedded sub-documents is useful, but perhaps even more useful is to embed multiple sub-documents under a single key. In other words, a property whose 24 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
35. value is a list or array of sub-documents. In MongoDB, this is a legal and very useful construct. This is a very natural way to model one-to-many relationships, or parent- child relationships. Consider the example of a “user” document which can reference multiple email addresses for that user. In the relational model, this would typically be achieved with two tables—one for users, and one for the email addresses associated with each user. A join query could then be used to retrieve a user along which each of its email addresses. In MongoDB, a natural approach to model a one-to-many relationship would be to simply have a property “emails” on the user document, the value of which is an array containing sub-documents, each representing an associated email account. For exam- ple: # A user document demonstrating one-to-many relationships using embedding # Here we map multiple email addresses (along with whether or not the email # is the user's primary email address) to a single user. user_doc = { "username":"foouser", "emails":[ { "email":"foouser1@example.com", "primary":True }, { "email":"foouser2@example2.com", "primary":False }, { "email":"foouser3@example3.com", "primary":False } ] } Not only does this work, but MongoDB has some specific features to help working with this type of embedded structure. Just as you can query for documents by the value of sub-documents directly embedded in the top-level document, documents can also be located by the value of sub-documents embedded in arrays. To do this, simply use the same dot (“.”) notation, as described earlier in this section. MongoDB transparently searches through arrays for sub-documents. Returning to our earlier example of a single user with multiple email addresses, consider the following code: # A user document demonstrating one-to-many relationships using embedding user_doc = { "username":"foouser", "emails":[ { "email":"foouser1@example.com", "primary":True }, A Uniquely Document-Oriented Pattern: Embedding | 25 www.it-ebooks.info
36. { "email":"foouser2@example2.com", "primary":False }, { "email":"foouser3@example3.com", "primary":False } ] } # Insert the user document dbh.users.insert(user_doc, safe=True) # Retrieve the just-inserted document via one of its many email addresses user_doc_result = dbh.users.find_one({"emails.email":"foouser1@example.com"}) # Assert that the original user document and the query result are the same assert user_doc == user_doc_result In addition to MongoDB understanding lists of sub-documents to enable querying for embedded values via the dot notation, there are also useful update modifiers. $pull, $push and their variants are the most helpful, enabling atomic append and removal of sub-documents to and from embedded lists. Consider the case where a user no longer wishes a particular email address to be linked to his or her account. The naive way to remove that email address from their user document would be to first query for their user document, modify it in your application code so it no longer contains the removed email address, and then send an update query to the database. Not only is this cum- bersome, it also introduces a race condition, as the underlying user document may have been modified by another process in between your read and write: # Naive method to remove an email address from a user document # Cumbersome and has a race condition user_doc = { "username":"foouser", "emails":[ { "email":"foouser1@example.com", "primary":True }, { "email":"foouser2@example2.com", "primary":False }, { "email":"foouser3@example3.com", "primary":False } ] } # Insert the user document dbh.users.insert(user_doc, safe=True) # Retrieve the just-inserted document via username user_doc_result = dbh.users.find_one({"username":"foouser"}) # Remove the "foouser2@example2.com" email address sub-document from the embedded list del user_doc_result["emails"][1] 26 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
37. # Now write the new emails property to the database # May cause data to be lost due to the race between read and write dbh.users.update({"username":"foouser"},{"$set":{"emails":user_doc_result}}, safe=True) The three most common operations on sub-documents embedded in a list property are: Deletion, insertion and modification. Each of these can be performed atomically with the provided update modifiers. First let’s demonstrate the use of $pull to remove the sub-document matching “foouser2@example2.com” in a simple and race-free way: # Atomically remove an email address from a user document race-free using the # $pull update modifier user_doc = { "username":"foouser", "emails":[ { "email":"foouser1@example.com", "primary":True }, { "email":"foouser2@example2.com", "primary":False }, { "email":"foouser3@example3.com", "primary":False } ] } # Insert the user document dbh.users.insert(user_doc, safe=True) # Use $pull to atomically remove the "foouser2@example2.com" email sub-document dbh.users.update({"username":"foouser"}, {"$pull":{"emails":{"email":"foouser2@example2.com"}}}, safe=True) In this example, $pull is used to match an embedded document with "email":"foouser2@example2.com" in the “emails” field. $pull will remove the entire document from the array in an atomic fashion, meaning there is no opportunity for a race condition. You can also use query modifiers with $pull, for example to remove all sub-documents with a “primary” value that is not equal to True, you could write the following: # Use $pull to atomically remove all email sub-documents with primary not equal to True dbh.users.update({"username":"foouser"}, {"$pull":{"emails":{"primary":{"$ne":True}}}, safe=True) The full range of query modifiers (see table in Chapter 2) are available for use, including $gt, $lt and so on. Additionally, $pull can be used with arrays containing atoms (num- bers, strings, dates, ObjectIDs etc). In other words, $pull doesn’t work only with em- bedded documents—if you store a list of primitive types in an array, you can remove elements atomically with $pull too. A Uniquely Document-Oriented Pattern: Embedding | 27 www.it-ebooks.info
38. The $push update modifier is used to atomically append an element to an array. At the time of writing, $push can only support adding items to the end of the array—there is no update modifier to add an element to the beginning of an array, or to insert it at an arbitrary index. $push is simple to use, because, unlike $pull, it does not take any field match or conditional arguments. For example, to atomically add a new email address to our user document, we could use the following query: # Use $push to atomically append a new email sub-document to the user document new_email = {"email":"fooemail4@exmaple4.com", "primary":False} dbh.users.update({"username":"foouser"}, {"$push":{"emails":new_email}}, safe=True) The final case is updating an existing sub-document in-place. This can be achieved using what is called the “positional” operator. The positional operator is represented by the dollar sign (“$”). Basically, it is replaced by the server with the index of the item matched by the document spec. For example, suppose we wish to make our user docu- ment’s “foouser2@example2.com” email address primary. We could issue the follow- ing update query to modify it in-place: # Demonstrate usage of the positional operator ($) to modify # matched sub-documents in-place. user_doc = { "username":"foouser", "emails":[ { "email":"foouser1@example.com", "primary":True }, { "email":"foouser2@example2.com", "primary":False }, { "email":"foouser3@example3.com", "primary":False } ] } # Insert the user document dbh.users.insert(user_doc, safe=True) # Now make the "foouser2@example2.com" email address primrary dbh.users.update({"emails.email":"foouser2@example2.com"}, {"$set":{"emails.$.primary":True}}, safe=True) # Now make the "foouser1@example.com" email address not primary dbh.users.update({"emails.email":"foouser1@example.com"}, {"$set":{"emails.$.primary":False}}, safe=True) Note that the $ operator cannot be used with upserts (see section on upserts later in this chapter) additionally it only works with the first matched element. 28 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
39. When working with embedding, it is important to be aware of the performance char- acteristics of documents and sub-documents in MongoDB. First and foremost, when a document is fetched from the database to answer a query, the entire document— including any and all embedded sub-documents—is loaded into memory. This means that there is no extra cost (aside from the additional network and decode/encode CPU overhead incurred by a larger result set) to fetch embedded data. Once the top-level document has been retrieved, its sub-documents are immediately avaialble, too. Con- trast this with a relational schema design utilizing joins, where the database may need to read from one or more additional tables to fetch associated data. Depending on the situation, these joins could impact query performance considerably. Secondly, it is also very important to be aware that there is a size limit on documents in MongoDB. Additionally, the document size limit has been increased over successive major MongoDB releases. In MongoDB 1.4.x and 1.6.x, the maximum document size was 4MB but in 1.8.x it was increased to 16MB. One can expect that this limit may continue to increase—perhaps eventually to be arbitrarily large—but for now, keep in mind that documents have a finite size when modeling your data. In practice, it is rare to reach even a 4MB document size, unless the design is such that documents continue to grow over time. For example, a scenario where new properties are created on an hourly or daily basis. In such cases, it is wise to ensure there is some application logic to handle purging old/expired embedded sub-documents to prevent the limit being hit. Another example would be building a document publishing platform which embedded every single document posted by a user as a sub-document inside of the user document. While performance would be excellent since a single query for the user document could retieve all their published documents in a single shot, it is quite likely that some users would eventually publish more than 16MB of content. Hence there is often a judgement call to be made when designing MongoDB schemas: To embed, or not to embed. The alternative to embedding is storing the documents in a separate collection and performing a join in your own application code, by querying twice or more. Usually many-to-many relationships are modeled in this way, while one-to-many relationships are embedded. Fast Lookups: Using Indexes with MongoDB The role of indexes in MongoDB is very similar to their role in traditional RDBMS such as MySQL, PostgreSQL, etc. MongoDB offers two kinds of indexes out-of-the-box: Btree indexes and geospatial indexes. The btree indexes in MongoDB are much the same as the equivalents in MySQL or PostgreSQL. When in a relational system you would put an index on a column to get fast lookups on that field, you do an analogous thing in MongoDB by placing an index on a particular property in a collection. Just as Fast Lookups: Using Indexes with MongoDB | 29 www.it-ebooks.info
40. with an RDBMS, MongoDB indexes can span multiple fields (a.k.a. compound indexes) —useful if you know in advance that you will be querying based on the value of more than a single property. A compound index would be useful for example if you were querying documents by first name and last name. In MongoDB, btree indexes can have a “direction”. This direction is only useful in the case of compound indexes, where the index direction should match the sort direction or range query direction for optimal performance. For example, if you are querying a range (say, A through C) on first name and last name and then sorting in ascending order on last name, your compound index direction should also be ascending. Using a btree index will incur a performance hit on writes, as the database must now update the index in addition to the data. For this reason, it is wise to choose your indexes carefully. Avoid superfluous indexes if at all possible. Indexes also take up valuable storage—not so much of an issue with on-disk space today given the low price-per- terrabyte—but in memory, too. Your database will run fastest when it resides entirely in memory, and indexes can considerably add to its size. It is a classic Computer Science time vs. space tradeoff scenario. MongoDB btree indexes can also be used to enforce a unique constraint on a particular property in a collection. By default, the _id primary key property has a unique index created in MongoDB. The unique constraint will prevent the protected property from ever having a duplicate value within the collection. This can be useful for values which are expected to be globally unique in the collection—a common example being user- names. Beware of over-reliance on this feature, however, as in the current implemen- tation of sharding, unique indexes are supported only on the _id property—otherwise they are not enforced globally across the cluster. Btree indexes also transparently support indexing multi-value properties, that is, prop- erties where the value is an array. Each item in the array will be properly stored in the index to enable fast retrieval of the parent document. This can be useful for performant implementations of tagging, where each tag is stored as a string inside a “tags” list property on the document. Lookups for documents matching one or more of those tags (potentially using the $in query operator) will then be looked up in the “tags” index. Furthermore, btree indexes are equally well supported when placed on embedded sub- documents. If, for example, you store email addresses as embedded sub-documents, and you wish to be able to look up by the value of the email address using an index, MongoDB allows this. Hence the following document and query could take advantage of an index: user_doc = { "username":"foouser", "emails":[ { "email":"foouser1@example.com", "primary":True }, { "email":"foouser2@example2.com", 30 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
41. "primary":False }, { "email":"foouser3@example3.com", "primary":False } ] } dbh.users.insert(user_doc) # If we place an index on property "emails.email", # e.g. dbh.users.create_index("emails.email") # this find_one query can use a btree index user = dbh.users.find_one({"emails.email":"foouser2@example2.com"}) Btree indexes in MongoDB are also important when performing server-side sorting of results. Without an index on the property you are sorting by, MongoDB will run out of memory when trying to sort anything greater than a relatively small results set (ap- prox. 4Mb at time of writing). If you expect that you will be sorting result sets larger than 4Mb, you should specify an index on the sort key. It is easy to underestimate this and find exceptions are being raised on queries against larger, real-world data which were not anticipated during development. To create an index with the PyMongo driver, use the Collection.create_index() method. This method can create single-key indexes or compound indexes. For a single- key index, only the key needs to be provided. A compound index is slightly more com- plicated—a list of 2-tuples (key, direction) must be supplied. For example to create an index on the username property of a collection called users, you could write the following: # Create index on username property dbh.users.create_index("username") To create a compound index, for example on the first_name and last_name, with an ascending direction, you could specify: # Create a compound index on first_name and last_name properties # with ascending index direction dbh.users.create_index([("first_name", pymongo.ASCENDING), ("last_name", pymongo.ASCENDING)]) Indexes in MongoDB each have names. By default, MongoDB will generate a name, but you may wish to give a custom name—particularly for compound indexes where the generated names are not especially readable by humans. To give a custom name during creation, supply the name= parameter to the create_index() method: # Create a compound index called "name_idx" on first_name and last_name properties # with ascending index direction dbh.users.create_index([ ("first_name", pymongo.ASCENDING), ("last_name", pymongo.ASCENDING) ], name="name_idx") Fast Lookups: Using Indexes with MongoDB | 31 www.it-ebooks.info
42. It should be noted that index creation locks the database by default. For large collec- tions, index creation can be time consuming. To help mitigate the impact of these operations on live, production databases, MongoDB is capable of building indexes in the background, without blocking database access. Building an index in the back- ground may take slightly longer, and will still cause additional load on the system, but the database should otherwise remain available. To specify that an index should be built in the background, pass the background=True parameter to the create_index() method: # Create index in the background # Database remains usable dbh.users.create_index("username", background=True) As mentioned earlier in this section, MongoDB btree indexes can be used to enforce a uniqueness constraint on a particular property. Unique constraints can be applied to both single-key indexes and compound indexes. To create an index with a unique constraint, simply pass the unique=True parameter to the create_index() method: # Create index with unique constraint on username property dbh.users.create_index("username", unique=True) Be aware that unique indexes in MongoDB do not function exactly the same as indexes in RDBMS systems. In particular, a document with a missing property will be added to the index as if it the value of that property were null. This means that when a unique constraint is added to a btree index in MongoDB, the database will prevent you from having multiple documents in the collection which are missing the indexed property. For example, if you have created a unique index for the username property in a users collection, only one document in that collection may be permitted to lack a username property. Writes of additional documents without a username property will raise an exception. If you try to add a unique index to a collection which already contains duplicates on the specified property, MongoDB will (unsurprisingly) raise an exception. However, if you don’t mind throwing away duplicate data, you can instruct MongoDB to drop all but the first document it finds using the dropDups or drop_dups parameter: # Create index with unique constraint on username property # instructing MongoDB to drop all duplicates after the first document it finds. dbh.users.create_index("username", unique=True, drop_dups=True) # Could equally be written: # dbh.users.create_index("username", unique=True, dropDups=True) Over time, your schema may evolve and you may find that a particular index is no longer needed. Fortunately, indexes are easy to remove in MongoDB. The Collec tion.drop_index() method deletes one index at a time. If you created your index with a custom name (as described above), you must supply this same name to the drop_index() method in order to delete it. For example: # Create index on username property called "username_idx" dbh.users.create_index("username", name="username_idx") # Delete index called "username_idx" dbh.users.drop_index("username_idx") 32 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
43. If, on the other hand, you did not give your index a custom name, you can delete by passing the original index specifier. For example: # Create a compound index on first_name and last_name properties # with ascending index direction dbh.users.create_index([("first_name", pymongo.ASCENDING), ("last_name", pymongo.ASCENDING)]) # Delete this index dbh.users.drop_index([("first_name", pymongo.ASCENDING), ("last_name", pymongo.ASCENDING)]) All indexes in a collection can be dropped in a single statement using the Collec tion.drop_indexes() method. If you wish to programatically inspect the indexes on your collections from Python, you can use the Collection.index_information() method. This returns a dictionary in which each key is the name of an index. The value associated with each key is an additional dictionary. These second-level dictionaries always contain a special key called key, which is an entry containing the original index specifier—including index direction. This original index specifier was the data passed to the create_index() method when the index was first created. The second-level dictionaries may also con- tain additional options such as unique constraints and so on. Location-based Apps with MongoDB: GeoSpatial Indexing As mentioned in the previous section on indexes, MongoDB has support for two kinds of index: Btree and geospatial. Btree indexes have been covered quite thoroughly in the preceeding section, however we have not yet described GeoSpatial indexes. First of all, let us discuss why geospatial indexing might be useful at all. Many apps today are being built with the requirement of location-awareness. Typically this trans- lates into features where points of interest (POI) near a particular user location may be rapidly retrieved from a database. For example, a location-aware mobile app might wish to quickly fetch a list of nearby coffeeshops, based upon the current GPS co- ordinates. The complicating issue, fundamentally, is that the world is both quite large and quite full of interesting points—and so to try to answer such a query by iterating through the entire list of all POIs in the world to find ones which are nearby would take an unacceptably long time. Hence the need for some sort of GeoSpatial indexing, to speed up these searches. Fortunately for anybody tasked with building location-aware applications, MongoDB is one of the rare few databases with out-of-the-box support for geospatial indexing. MongoDB uses geohashing, a public domain algorithm developed by Gustavo Nie- meyer, which translates geographic proximity into lexical proximity. Hence, a database supporting range queries (such as MongoDB) can be efficiently used to query for points near and within bounds. Location-based Apps with MongoDB: GeoSpatial Indexing | 33 www.it-ebooks.info
44. It should be noted that at present, MongoDB’s geospatial indexing support is limited purely to point-based querying. The supplied operators can only be used for finding individual points—not routes or sub-shapes. MongoDB provides the $near and $within operators which constitute the primary means for performing geospatial queries in the system. Using $near, you can efficiently sort documents in a collection by their proximity to a given point. The $within operator allows you to specify a bounds for the query. Supported boundary definitions include $box for a rectangular shape, $circle for a circle. In MongoDB 1.9 and up, the $poly gon operator allows for convex and concave polygon boundaries. Before you can use the geospatial queries, you must have a geospatial index. In Mon- goDB versions up to and including 1.8.x, geospatial indexes are limited to a single index per collection. This means that each document can have only one location property queried efficiently by MongoDB. This can have some important implications for schema design which is why it is good to know from the outset. Geospatial indexes by default limit acceptable values for the location property on documents to those within GPS. That is, co-ordinates must be in the range -180 +180. If you have co-ordinates outside of this range, MongoDB will raise an exception when you attempt to create the geospatial index on the colleciton. If you wish to index values outside of the range of regular GPS, you can specify this at index creation time. The location property on your documents must be either an array or sub-document where the first two items are the x and y co-ordinates to be indexed. The order of the co-ordinates (whether x,y or y,x) does not matter so long as it is consistent on all docu- ments. For example, your document could look like any of the following: # location property is an array with x,y ordering user_doc = { "username":"foouser", "user_location":[x,y] } # location property is an array with y,x ordering user_doc = { "username":"foouser", "user_location":[y,x] } import bson # location property is a sub-document with y,x ordering loc = bson.SON() loc["y"] = y loc["x"] = x user_doc = { "username":"foouser", "user_location":loc } 34 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
45. import bson # location property is a sub-document with x,y ordering loc = bson.SON() loc["x"] = x loc["y"] = y user_doc = { "username":"foouser", "user_location":loc } Note that in Python the default dictionary type (dict class), order is not preserved. When using location in a sub-document from Python, use a bson.SON object instead. bson.SON comes with the PyMongo driver, and is used in exactly the same way as Python’s dict class. Once the documents in your collection have their location properties correctly formed, we can create the geospatial index. As with btree indexes, geospatial indexes in Mon- goDB are created with PyMongo’s Collection.create_index() method. Due to the one- geospatial-index-per-collection limitation in MongoDB versions up to and including 1.8.x, if you are planning to query by other properties in addition to the location prop- erty, you can make your geospatial index a compound index. For exmaple, if you know that you will be searching your collection by both “username” and “user_location” properties, you could create a compound geo index across both fields. This can help to work around the single geospatial index limitation in many cases. Returning to our example of documents in a collection called “users” with the location property being “user_location”, we would create a geospatial index with the following statement: # Create geospatial index on "user_location" property. dbh.users.create_index([("user_location", pymongo.GEO2D)]) To create a compound geospatial index which would let us query efficiently on location and username, we could issue this statement: # Create geospatial index on "user_location" property. dbh.users.create_index([("user_location", pymongo.GEO2D), ("username", pymongo.ASCENDING)]) Now that we have geospatial indexes available, we can try out some efficient location- based queries. The $near operator is pretty easy to understand, so we shall start there. As has already been explained, $near will sort query results by proximity to specified point. By default, $near will try to find the closest 100 results. An important performance consideration which is not mentioned clearly in the official MongoDB documentation is that when using $near, you will almost always want to specify a maximum distance on the query. Without a clamp on the maximum distance, in order to return the specified number of results (default 100) MongoDB has to search through the entire database. This takes a lot of time. In most cases, a max distance of Location-based Apps with MongoDB: GeoSpatial Indexing | 35 www.it-ebooks.info
46. around 5 degrees should be sufficient. Since we are using decimal degrees (a.k.a GPS) co-ordinates, the units of max distance is degrees. 1 degree is roughly 69 miles. If you only care about a relatively small set of results (for example, the nearest 10 coffee shops), limiting the query to 10 results should also aid performance. Let’s start with an example of finding the nearest 10 users to the point 40, 40 limiting to a max distance of 5 degrees: # Find the 10 users nearest to the point 40, 40 with max distance 5 degrees nearest_users = dbh.users.find( {"user_location": {"$near" : [40, 40], "$maxDistance":5}}).limit(10) # Print the users for user in nearest_users: # assume user_location property is array x,y print "User %s is at location %s,%s" %(user["username"], user["user_location"][0], user["user_location"[1]) Next let us try using the $within geospatial operator to find points within a certain boundary. This can be useful when searching for POI’s in a specific county/city or even well-defined neighbourhood within a city. In the real world, these boundaries are fuzzy and changing constantly, however there are good enough databases available for them to be useful. To specify a rectangle to search within, you simply provide the lower-left and top-right co-ordinates as elements in an array. For example: box = [[50.73083, -83.99756], [50.741404, -83.988135]] We could query for points within this bound by using the following geospatial query: box = [[50.73083, -83.99756], [50.741404, -83.988135]] users_in_boundary = dbh.users.find({"user_location":{"$within": {"$box":box}}}) To specify a citcle to search within, you just supply the center point and the radius. As with $maxDistance mentioned previously, the units of the radius are degrees. Here is how we could make a geospatial lookup for 10 users within a radius of 5 degress cen- tered at the point 40, 40: users_in_circle = dbh.users.find({"user_location":{"$within":{"$center":[40, 40, 5]}}}).limit(10) Notice that with the circle boundary using $center, we pass an array, the first two values of which are the co-ordinates of the center and the third parameter is the radius (in degrees). All the queries we’ve mentioned so far which make use of a geospatial index actually are not entirely accurate. This is because they use a flat earth model where each arc degree of latitude and longitude translates to the same distance everywhere on the earth. In reality, the earth is a sphere and so these values differ depending upon where you are. Fortunately, MongoDB in 1.8.x and up implements a spherical model of the earth for geospatial queries. 36 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
47. The new spherical model can be used by employing the $nearSphere and $circle Sphere variants on the $near and $circle operators. MongoDB’s spherical model has a few extra caveats. First and foremost, you must use (longitude, latitude) ordering of your co-ordinates. While there are many other application and formats which use the (latitude, longitude) ordering, you should be careful to re-order to use with MongoDB’s spherical model. Secondly, unlike the $near and $center operators we just described, the units for distances with $nearSphere and $centerSphere are always expressed in radians. This includes when using $maxDistance with $nearSphere or $centerSphere. Luckily, it is not difficult to convert from a more humanly-understandable unit such as kilometers to radians. To translate from kilometers to radians, simply divide the kilometer value by the radius of the earth which is approximately 6371 km (or 3959 miles). To demonstrate, let’s try our earlier example of finding the 10 users nearest to the point 40,40 with a max distance of 5 km—but this time using the spherical model: # Find the 10 users nearest to the point 40, 40 with max distance 5 degrees # Uses the spherical model provided by MongoDB 1.8.x and up earth_radius_km = 6371.0 max_distance_km = 5.0 max_distance_radians = max_distance_km / earth_radius_km nearest_users = dbh.users.find( {"user_location": {"$nearSphere" : [40, 40], "$maxDistance":max_distance_radians}}).limit(10) # Print the users for user in nearest_users: # assume user_location property is array x,y print "User %s is at location %s,%s" %(user["username"], user["user_location"][0], user["user_location"[1]) Code Defensively to Avoid KeyErrors and Other Bugs One of the tradeoffs of a document-oriented database versus a relational database is that the database does not enforce schema for you. For this reason, when working with MongoDB you must be vigilant in your handling of database results. Do not blindly assume that results will always have the properties you expect. Check for their presence before accessing them. Although Python will generally raise a KeyError and stop execution, depending on your application this still may result in loss of data integrity. Consider the case of updating every document in a collection one- by-one—a single unforeseen KeyError could leave the database in an inconsistent state, with some documents having been updated and others not. For example, all_user_emails = [] for username in ("jill", "sam", "cathy"): user_doc = dbh.users.find_one({"username":username}) # KeyError will be raised if any of these does not exist dbh.emails.insert({"email":user_doc["email"]}) Code Defensively to Avoid KeyErrors and Other Bugs | 37 www.it-ebooks.info
48. Sometimes you will want to have a default fallback to avoid KeyErrors should a docu- ment be returned which is missing a property required by your program. The Python dict class’ get method easily lets you specify a default value for a property should it be missing. Often it makes sense to use this defensively. For example, imagine we have a collection of user documents. Some users have a “score” property set to a number, while others do not. It would be safe to have a default fallback of zero (0) in the case that no score property is present. We can take a missing score to mean a zero score. The dict class’s get method lets us do this easily: total_score = 0 for username in ("jill", "sam", "cathy"): user_doc = dbh.users.find_one({"username":username}) total_score += user_doc.get("score", 0) This approach can also work well when looping over embedded lists. For example, to defensively handle the case where a document representing a particular product does not yet have a list of suppliers embedded (perhaps because it is not yet on the market, or is no longer being produced) you might write code like this: # Email each supplier of this product. # Default value is the empty list so no special casing # is needed if the suppliers property is not present. for supplier in product_doc.get("suppliers", []): email_supplier(supplier) MongoDB also makes no guarantees about the type of a property’s value on a given document. In most RDBMS implementations (the notable exception I’m aware of being SQLite) the database will quite rigorously enforce column types. If you try to insert a string into an integer column, the database will reject the write. MongoDB, on the other hand, will only in exceptional circumstances reject such writes. If you set the value of a property on one document to be a string and on another docu- ment in the same collection set the value of that property to a number, it will very happily store that. # Perfectly legal insert - MongoDB will not complain dbh.users.insert({"username":"janedoe"}) # Also perfectly legal - MongoDB will not complain dbh.users.insert({"username":l337}) When you couple this with Python’s willingness to let you forgo explicitly typing your variables, you can soon run into trouble. Perhaps the most common scenario is when writing inputs from Web applications to the database. Most WSGI-based Python frameworks will send you all HTTP POST and GET parameter values as strings— regardless of whether or not they are in fact strings. Thus it is easy to insert or update a numeric property with a value that is a string. The best way, of course, to avoid errors of this nature is to prevent the wrong type of data 38 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
49. ever being written to the database in the first place. Thus, in the context of a Web application, validating and/or coercing the types of any inputs to write queries before issuing them is strongly advised. You may consider using the FormEncode or Colander Python libraries to help with such validation. Update-or-Insert: Upserts in MongoDB A relatively common task in a database-powered application is to update an existing entry, or if not found insert it as a new record. MongoDB conveniently supports this as a single operation, freeing you from having to implement your own “if-exists-update- else-insert” logic. 10gen refer to this type of write operation as an “upsert”. In PyMongo, there are three possible methods one can use to perform an upsert. These are Collection.save(), Collection.update() and Collection.find_and_modify(). We shall start by describing Collection.save() as it is the most straight forward method. In the earlier section “Inserting a Document into a Collection” we used the Collec tion.insert() method to write a new document to the collection. However, we could have just as easily used the save() method. save() offers almost identical functionality to insert() with the following exceptions: save() can perform upserts and save() can- not insert multiple documents in a single call. save() is quite easy to understand. If you pass it a document without an _id property it will perform an insert(), creating a brand new document. If, on the other hand, you pass it a document which contains an _id property it will update the existing document corresponding to that _id value, overwriting the already-present document with the document passed to save(). This is the essence of an upsert: If a document already exists, update it. Otherwise create a new document. save() can be useful because it supports both writing new documents and modifying existing documents, most likely ones retrieved from MongoDB via a read query. Having a single method which is capable of both modes of operation reduces the need for conditional handling in your client code, thus simplifying your program. More useful, perhaps, is the “upsert=True” parameter which may be passed to Collec tion.update(). As has been discussed in the section “Updating Documents in a Col- lection” and is further described in the section “MongoDB Update Modifiers”, the update() method supports the use of “update modifiers”. These rich operators enable you to perform writes more complex than the basic “overwrite but keep existing _id” semantics of the save() method. For example, imagine you are writing a method, edit_or_add_session(). This method either edits an existing document, or inserts a new one. Furthermore, semantics of the method dictate that the method will always be called with a session_id, but that the session_id may or may not already be present in the database. The naive implementa- Update-or-Insert: Upserts in MongoDB | 39 www.it-ebooks.info
50. tion would first query to see whether a session document already existed, then condi- tionally either insert a new session document or update the existing document: # Naive, bad implementation without upsert=True def edit_or_add_session(description, session_id): # We must query first, becase we don't know whether this session_id already exists. # If we attempt to update a non-existent document, no write will occur. session_doc = dbh.sessions.find_one({"session_id":session_id}) if session_doc: dbh.sessions.update({"session_id":session_id}, {"$set":{"session_description":description}}, safe=True) else: dbh.sessions.insert({"session_description":description, "session_id":session_id}, safe=True) However, by employing the upsert feature of Collection.update(), this can be imple- mented in a single method call—simplifying the code considerably and eliminating the need for an extra read query: # Good implementation using upsert=True def edit_or_add_session(description, session_id): dbh.sessions.update({"session_id":session_id}, {"$set":{"session_description":description}}, safe=True, upsert=True) Note that we could not have implemented the above semantics using Cursor.save() because we are testing for existence on the property “session_id” rather than “_id”. Recall that the save() upsert method only works with “_id”. The trick to understanding upserts with the update() method is to consider the two execution cases seperately. In the case that the document already exists, then the update document will be processed normally—just as with a regular update() call without the “upsert=True” parameter. However, in the case that the document does not already exist, the document written (upserted) will match both the document spec supplied as the first argument to the update() call and the update document with any modifiers it contains. In other words, the observed behaviour is that the document is first created with the properties specified in the document spec—in this case, "session_id":ses sion_id—and then the update document is executed against that. That may not accu- rately reflect what is happening internally in the daemon or driver, but that is equivalent to whatever does go on. Atomic Read-Write-Modify: MongoDB’s findAndModify We’ve already introduced the atomic update modifiers supported by MongoDB. These are very powerful and enable race-free write operations of many kinds—including array manipulation and increment/decrement. However, it is often necessary to be able to modify the document atomically, and also return the result of the atomic operation— in a single step. 40 | Chapter 3: Common MongoDB and Python Patterns www.it-ebooks.info
51. For example, imagine a billing system. Each user document has an “account_balance” property. There may be writes which alter the account balance—let’s say an account top-up event which adds money to the account, and a purchase action which takes money from the account. Consider the following implementation: # User X adds $20 to his/her account, so we atomically increment # account_balance dbh.users.update({"username":username}, {"$inc":{"account_balance":20}}, safe=True) # Fetch the updated account balance to display to user new_account_balance = dbh.users.find_one({"username":username}, {"account_balance":1})["account_balance"] This will work fine assuming no other writes to the account balance occur between the write and read operations. There is an obvious race condition. If a purchase action were to take place between the balance update and the balance read, the user may be dis- pleased to be presented with a post-payment balance of less than what they expected! # User X adds $20 to his/her account, so we atomically increment # account_balance dbh.users.update({"username":username}, {"$inc":{"account_balance":20}}, safe=True) # In the meantime, in another thread or process, there is a payment operation, # which decrements the account balance: dbh.users.update({"username":username}, {"$dec":{"account_balance":5}}, safe=True) # Fetch the updated account balance to display to user new_account_balance = dbh.users.find_one({"username":username}, {"account_balance":1})["account_balance"] What you want in this kind of situation is a way to update the account balance and return the new value in a single, atomic operation. MongoDB’s findAndModify com- mand allows you to do just this. PyMongo provides a wrapper around findAndModify in the Collection.find_and_modify() method. Using this method, we can rewrite the code to a single, atomic operation: # User X adds $20 to his/her account, so we atomically increment # account_balance and return the resulting document ret = dbh.users.find_and_modify({"username":username}, {"$inc":{"account_balance":20}}, safe=True, new=True) new_account_balance = ret["account_balance"] Fast Accounting Pattern Many of the applications people are building today are realtime with very large data sets. That is to say, users expect changes they make to be reflected within the application instantly. For example if a user wins a new high score in a multiplayer game, they expect the high score table in the game to be updated immediately. However, it may not be a single high score table which must be updated. Perhaps you are also ranking by high score this week, or this month, or even this year. Furthermore, as the application de- veloper you may wish to keep a detailed log of each change—including when it occured, Fast Accounting Pattern | 41 www.it-ebooks.info