A MongoDB driver for Ruby
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Moped is a MongoDB driver for Ruby, which exposes a simple, elegant, and fast API.

session = Moped::Session.new %w[]
session.use "echo_test"

session.with(safe: true) do |safe|
  safe[:artists].insert(name: "Syd Vicious"

session[:artists].find(name: "Syd Vicious").
    :$push => { instruments: { name: "Bass" } }


  • Automated replica set node discovery and failover.
  • No C or Java extensions
  • No external dependencies
  • Simple, stable, public API.

Unsupported Features

  • GridFS
  • Map/Reduce

These features are possible to implement, but outside the scope of Moped's goals. Consider them perfect opportunities to write a companion gem!

Project Breakdown

Moped is composed of three parts: an implementation of the BSON specification, an implementation of the Mongo Wire Protocol, and the driver itself. An overview of the first two follows now, and after that more information about the driver.


Moped::BSON is the namespace for Moped's BSON implementation. It's implemented in pure (but fast) ruby. The public entry point into the BSON module is BSON::Document, which is just subclass of Hash, but exposes two class methods: serialize and deserialize. serialize accepts a BSON::Document (or Hash) and returns the serialized BSON representation. deserialize does the opposite: it reads data from an IO-like input and returns a deserialized BSON::Document.


The ObjectId class is used for generating and interacting with Mongo's ids.

id = Moped::BSON::ObjectId.new # => 4f8583b5e5a4e46a64000002
id.generation_time # => 2012-04-11 13:14:29 UTC
id == Moped::BSON::ObjectId.from_string(id.to_s) # => true
new Creates a new object id.
from_string Creates a new object id from an object id string.
from_time Creates a new object id from a time.
legal? Validates an object id string.


The Code class is used for working with javascript on the server.

Moped::BSON::Code.new("function () { return this.name }")
Moped::BSON::Code.new("function (s) { return s.prefix + this.name }",
  prefix: "_"


The Binary class allows you to persist binary data to the server, and supports the following types: :generic, :function, :old, :uuid, :md5, and :user. Note that :old is deprecated, but still present to support legacy data.

Moped::BSON::Binary.new(:md5, Digest::MD5.digest(__FILE__))


Moped::Protocol is the namespace for Moped's implementation of the Mongo Wire Protocol. Its public API consists of classes representing each type of message in the protocol: Delete, GetMore, Insert, KillCursors, Query, Reply, Update, and a convenience class Command.

You should never have to worry about protocol objects, but more details can be found in the API documentation if you're interested.

Driver API

This is the core, public API for Moped. It lives almost entirely in four classes:

  • Session: the root object for all interactions with mongo (c.f., db in the mongo shell).
  • Collection: for working with collections in the context of a session
  • Indexes: for manipulating and inspecting a collection's indexes
  • Query: for querying, as well as modifying existing data.

What follows is a "whirlwind" overview of the Moped driver API. For details on additional options, and more examples, use the generated API docs.



session = Moped::Session.new %w[]
session.use :moped_test
session.command ping: 1 # => {"ok"=>1.0}

session.with(safe: { w: 2, wtimeout: 5 }) do |safe_session|

session.with(database: "important_db", consistency: :strong) do |session|


use Set the current database
session.use :my_app_test
with Return or yield a copy of session with different options.
session.with(safe: true) { |s| ... }
session.with(database: "admin").command(...)
[] Choose a collection in the current database.
drop Drop the current database
command Run a command on the current database.
session.command(ping: 1)
login Log in to the current database.
session.login(username, password)
logout Log out from the current database.



users = session[:users]
users.find.count # => 0.0

users.indexes.create({name: 1}, {unique: true})

users.insert(name: "John")
users.find.count # => 1.0

users.insert(name: "John")
users.find.count # => 1.0

session.with(safe: true) do |session|
  session[:users].insert(name: "John")
end # raises Moped::Errors::OperationFailure


drop Drop the collection
indexes Access information about this collection's indexes
find Build a query on the collection
users.find(name: "John")
insert Insert one or multiple documents.
users.insert(name: "John")
users.insert([{name: "John"}, {name: "Mary"}])



session[:users].indexes.create(name: 1)
  { name: 1, location: "2d" },
  { unique: true }
session[:users].indexes[name: 1]
# => {"v"=>1, "key"=>{"name"=>1}, "ns"=>"moped_test.users", "name"=>"name_1" }

session[:users].indexes.drop(name: 1)
session[:users].indexes[name: 1] # => nil


[] Get an index by its spec.
indexes[id: 1]
create Create an index
indexes.create({name: 1}, {unique: true})
drop Drop one or all indexes
indexes.drop(name: 1)
each Yield each index
indexes.each { |idx| }



users = session[:users]

users.insert(name: "John")
users.find.count # => 1

users.find(name: "Mary").upsert(name: "Mary")
users.find.count # => 2

users.find.skip(1).limit(1).sort(name: -1).one
# => {"_id" => <...>, "name" => "John" }

scope = users.find(name: "Mary").select(_id: 0, name: 1)
scope.one # => {"name" => "Mary" }
scope.one # nil


limit Set the limit for this query.
skip Set the offset for this query.
sort Sort the results of the query
query.sort(name: -1)
distinct Get the distinct values for a field.
select Select a set of fields to return.
query.select(_id: 0, name: 1)
one/first Return the first result from the query.
each Iterate through the results of the query.
query.each { |doc| }
count Return the number of documents matching the query.
update Update the first document matching the query's selector.
query.update(name: "John")
update_all Update all documents matching the query's selector.
query.update_all(name: "John")
upsert Create or update a document using query's selector.
query.upsert(name: "John")
remove Remove a single document matching the query's selector.
remove_all Remove all documents matching the query's selector.


Here's a list of the exceptions generated by Moped.

Moped::Errors::ConnectionFailure Raised when a node cannot be reached or a connection is lost.
Note: this exception is only raised if Moped could not reconnect, so you shouldn't attempt to rescue this.
Moped::Errors::OperationFailure Raised when a command fails or is invalid, such as when an insert fails in safe mode.
Moped::Errors::QueryFailure Raised when an invalid query was sent to the database.
Moped::Errors::AuthenticationFailure Raised when invalid credentials were passed to `session.login`.
Moped::Errors::SocketError Not a real exception, but a module used to tag unhandled exceptions inside of a node's networking code. Allows you to `rescue Moped::SocketError` which preserving the real exception.

Other exceptions are possible while running commands, such as IO Errors around failed connections. Moped tries to be smart about managing its connections, such as checking if they're dead before executing a command; but those checks aren't foolproof, and Moped is conservative about handling unexpected errors on its connections. Namely, Moped will not retry a command if an unexpected exception is raised. Why? Because it's impossible to know whether the command was actually received by the remote Mongo instance, and without domain knowledge it cannot be safely retried.

Take for example this case:

session.with(safe: true)["users"].insert(name: "John")

It's entirely possible that the insert command will be sent to Mongo, but the connection gets closed before we read the result for getLastError. In this case, there's no way to know whether the insert was actually successful!

If, however, you want to gracefully handle this in your own application, you could do something like:

document = { _id: Moped::BSON::ObjectId.new, name: "John" }

rescue Moped::Errors::SocketError
  session["users"].find(_id: document[:_id]).upsert(document)

Replica Sets

Moped has full support for replica sets including automatic failover and node discovery.

Automatic Failover

Moped will automatically retry lost connections and attempt to detect dead connections before sending an operation. Note, that it will not retry individual operations! For example, these cases will work and not raise any exceptions:

session[:users].insert(name: "John")
# kill primary node and promote secondary
session[:users].insert(name: "John")
session[:users].find.count # => 2.0

# primary node drops our connection
session[:users].insert(name: "John")

However, you'll get an operation error in a case like:

# primary node goes down while reading the reply
session.with(safe: true)[:users].insert(name: "John")

And you'll get a connection error in a case like:

# primary node goes down, no new primary available yet
session[:users].insert(name: "John")

If your session is running with eventual consistency, read operations will never raise connection errors as long as any secondary or primary node is running. The only case where you'll see a connection failure is if a node goes down while attempting to retrieve more results from a cursor, because cursors are tied to individual nodes.

When two attempts to connect to a node fail, it will be marked as down. This removes it from the list of available nodes for :down_interval (default 30 seconds). Note that the :down_interval only applies to normal operations; that is, if you ask for a primary node and none is available, all nodes will be retried. Likewise, if you ask for a secondary node, and no secondary or primary node is available, all nodes will be retreied.

Node Discovery

The addresses you pass into your session are used as seeds for setting up replica set connections. After connection, each seed node will return a list of other known nodes which will be added to the set.

This information is cached according to the :refresh_interval option (default: 5 minutes). That means, e.g., that if you add a new node to your replica set, it should be represented in Moped within 5 minutes.


Moped is thread-safe -- depending on your definition of thread-safe. For Moped, it means that there's no shared, global state between multiple sessions. What it doesn't mean is that a single Session instance can be interacted with across threads.

Why not? Because threading is hard. Well, it's more than that -- though the public API for Moped is quite simple, MongoDB requires a good deal of complexity out of the internal API, specifically around replica sets and failover. We've decided that, for now, it's not worth making the replica set code thread-safe.

TL;DR: use one Moped::Session instance per thread.


Moped is tested against MRI 1.9.2, 1.9.3, 2.0.0, and JRuby (1.9).

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