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Ruby driver for MongoDB


This API documentation is available online at for all releases of the MongoDB Ruby driver. Please reference the exact version of the documentation that matches the release of the Ruby driver that you are using. Note that the Ruby Language Center for MongoDB has a link to API Documentation for the current release.

If you have the source, you can generate the matching documentation by typing

$ rake ydoc

Then open the file +ydoc/index.html+.


This is the 10gen-supported Ruby driver for MongoDB.

This documentation includes other articles of interest, include:

  1. A tutorial.
  2. Replica Sets in Ruby.
  3. Write Concern in Ruby.
  4. Tailable Cursors in Ruby.
  5. Read Preference in Ruby.
  6. GridFS in Ruby.
  7. Frequently Asked Questions.
  8. History.
  9. Release plan.
  10. Credits.

Here's a quick code sample. Again, see the MongoDB Ruby Tutorial for much more:

require 'rubygems'
require 'mongo'

@conn =
@db   = @conn['sample-db']
@coll = @db['test']

3.times do |i|
  @coll.insert({'a' => i+1})

puts "There are #{@coll.count} records. Here they are:"
@coll.find.each { |doc| puts doc.inspect }


Ruby Versions

The driver works and is consistently tested on Ruby 1.8.7 and 1.9.3 as well as JRuby 1.6.6.

Note that if you're on 1.8.7, be sure that you're using a patchlevel >= 249. There are some IO bugs in earlier versions.


The driver's gems are hosted at Make sure you're using the latest version of rubygems:

$ gem update --system

Then you can install the mongo gem as follows:

$ gem install mongo

The driver also requires the bson gem:

$ gem install bson

And for a significant performance boost, you'll want to install the C extensions:

$ gem install bson_ext

Note that bson_ext isn't used with JRuby. Instead, some native Java extensions are bundled with the bson gem. If you ever need to modify these extensions, you can recompile with the following rake task:

$ rake build:java

From the GitHub source

The source code is available at You can either clone the git repository or download a tarball or zip file. Once you have the source, you can use it from wherever you downloaded it or you can install it as a gem from the source by typing

$ rake gem:install

To install the C extensions from source, type this instead:

$ rake gem:install_extensions

That's all there is to it!


For extensive examples, see the MongoDB Ruby Tutorial.

Bundled with the driver are many examples, located in the "docs/examples" subdirectory. Samples include using the driver and using the GridFS class GridStore. MongoDB must be running for these examples to work, of course.

Here's how to start MongoDB and run the "simple.rb" example:

  $ cd path/to/mongo
  $ ./mongod run
  ... then in another window ...
  $ cd path/to/mongo-ruby-driver
  $ ruby docs/examples/simple.rb

See also the test code, especially test/test_db_api.rb.


The Ruby driver include two abstractions for storing large files: Grid and GridFileSystem. The Grid class is a Ruby implementation of MongoDB's GridFS file storage specification. GridFileSystem is essentially the same, but provides a more filesystem-like API and assumes that filenames are unique.

An instance of both classes represents an individual file store. See the API reference for details, and see examples/gridfs.rb for code that uses many of the Grid features (metadata, content type, seek, tell, etc).


  # Write a file on disk to the Grid
  file ='image.jpg')
  grid =
  id   = grid.put(file)

  # Retrieve the file
  file = grid.get(id)

  # Get all the file's metata


Thread Safety

The driver is thread-safe.

Connection Pooling

The driver implements connection pooling. By default, only one socket connection will be opened to MongoDB. However, if you're running a multi-threaded application, you can specify a maximum pool size and a maximum timeout for waiting for old connections to be released to the pool.

To set up a pooled connection to a single MongoDB instance:

@conn ="localhost", 27017, :pool_size => 5, :timeout => 5)

Though the pooling architecture will undoubtedly evolve, it currently owes much credit to the connection pooling implementations in ActiveRecord and PyMongo.


Certain Ruby application servers work by forking, and it has long been necessary to re-establish the child process's connection to the database after fork. But with the release of v1.3.0, the Ruby driver detects forking and reconnects automatically.

String Encoding

The BSON ("Binary JSON") format used to communicate with Mongo requires that strings be UTF-8 (

Ruby 1.9 has built-in character encoding support. All strings sent to Mongo and received from Mongo are converted to UTF-8 when necessary, and strings read from Mongo will have their character encodings set to UTF-8.

When used with Ruby 1.8, the bytes in each string are written to and read from Mongo as is. If the string is ASCII, all is well, because ASCII is a subset of UTF-8. If the string is not ASCII, it may not be a well-formed UTF-8 string.

Primary Keys

The _id field is a primary key. It is treated specially by the database, and its use makes many operations more efficient. The value of an _id may be of any type. The database itself inserts an _id value if none is specified when a record is inserted.

Primary Key Factories

A primary key factory is a class you supply to a DB object that knows how to generate _id values. If you want to control _id values or even their types, using a PK factory lets you do so.

You can tell the Ruby Mongo driver how to create primary keys by passing in the :pk option to the Connection#db method.

db ='dbname', :pk =>

A primary key factory object must respond to :create_pk, which should take a hash and return a hash which merges the original hash with any primary key fields the factory wishes to inject.

NOTE: if the object already has a primary key, the factory should not inject a new key; this means that the object may already exist in the database. The idea here is that whenever a record is inserted, the :pk object's +create_pk+ method will be called and the new hash returned will be inserted.

Here is a sample primary key factory, taken from the tests:

class TestPKFactory
  def create_pk(row)
    row['_id'] ||=

Here's a slightly more sophisticated one that handles both symbol and string keys. This is the PKFactory that comes with the MongoRecord code (an ActiveRecord-like framework for non-Rails apps) and the AR Mongo adapter code (for Rails):

class PKFactory
  def create_pk(row)
    return row if row[:_id]
    row.delete(:_id)      # in case it exists but the value is nil
    row['_id'] ||=

A database's PK factory object may be set either when a DB object is created or immediately after you obtain it, but only once. The only reason it is changeable at all is so that libraries such as MongoRecord that use this driver can set the PK factory after obtaining the database but before using it for the first time.

The DB Class

Strict mode

Each database has an optional strict mode. If strict mode is on, then asking for a collection that does not exist will raise an error, as will asking to create a collection that already exists. Note that both these operations are completely harmless; strict mode is a programmer convenience only.

To turn on strict mode, either pass in :strict => true when obtaining a DB object or call the :strict= method:

db ='dbname', :strict => true)
# I'm feeling lax
db.strict = false
# No, I'm not!
db.strict = true

The method DB#strict? returns the current value of that flag.



  • Cursors are enumerable (and have a #to_a method).

  • The query doesn't get run until you actually attempt to retrieve data from a cursor.

  • Cursors will timeout on the server after 10 minutes. If you need to keep a cursor open for more than 10 minutes, specify :timeout => false when you create the cursor.

Socket timeouts

The Ruby driver support timeouts on socket read operations. To enable them, set the :op_timeout option when you create a Mongo::Connection object.

If implementing higher-level timeouts, using tools like Rack::Timeout, it's very important to call Mongo::Connection#close to prevent the subsequent operation from receiving the previous request.

Test-Unit, Shoulda, and Mocha

Running the test suite requires test-unit, shoulda, and mocha. You can install them as follows:

$ gem install test-unit
$ gem install shoulda
$ gem install mocha

The tests assume that the Mongo database is running on the default port. You can override the default host (localhost) and port (Connection::DEFAULT_PORT) by using the environment variables MONGO_RUBY_DRIVER_HOST and MONGO_RUBY_DRIVER_PORT.


If you have the source code, you can run the tests. Skip this test with the C extension if you're running JRuby.

$ rake test:c

If you want to test the basic Ruby encoder, without the C extension, or if you're running JRuby:

$ rake test:ruby

These will run both unit and functional tests. To run these tests alone:

$ rake test:unit
$ rake test:functional

To run any individual rake tasks with the C extension enabled, just pass C_EXT=true to the task (don't do this with JRuby):

$ rake test:unit C_EXT=true

If you want to test replica set, you can run the following task:

$ rake test:rs

Release Notes





Copyright 2008-2010 10gen Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

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