Mongoid is customarily configured through a mongoid.yml file that specifies options and clients. The simplest configuration is as follows, which configures Mongoid to talk to a MongoDB server at “localhost:27017” and use the database named “mongoid”.

      database: mongoid
        - localhost:27017

The top level key in the configuration file, development in the above example, refers to the environment name which the application is executing in, i.e. development, test or production. The third level key, default in the above example, refers to the Mongo client name. Most applications will use a single client named default.

Rails Applications

If you are using Rails, you can have Mongoid generate a default configuration file for you by running the following command:

$ rails g mongoid:config

The configuration file will be placed in config/mongoid.yml.

Environment Determination

When Mongoid loads its configuration, it chooses the environment to use in the following order:

  • Rails.env if using Rails.
  • Sinatra::Base.environment if using Sinatra.
  • RACK_ENV environment variable.
  • MONGOID_ENV environment variable.

If you are not building a Rack based application and want to override the environment programatically, you can pass a second parameter to load! as follows:

Mongoid.load!("path/to/your/mongoid.yml", :production)

Depending on your version of Rails, you may also need to configure the ORM to be Mongoid in application.rb like so:

config.generators do |g|
  g.orm :mongoid

Anatomy of a Mongoid Config

Let’s have a look at a full-blown mongoid.yml and explain the full power of what Mongoid can do. The following configuration has explanation of the various options in the comments above each key:

  # Configure available database clients. (required)
    # Define the default client. (required)
      # A uri may be defined for a client:
      # uri: 'mongodb://'
      # Please see driver documentation for details. Alternatively, you can
      # define the following:
      # Define the name of the default database that Mongoid can connect to.
      # (required).
      database: my_db
      # Provide the hosts the default client can connect to. Must be an array
      # of host:port pairs. (required)
        # Change the default write concern. (default = { w: 1 })
          w: 1

        # Change the default read preference. Valid options for mode are: :secondary,
        # :secondary_preferred, :primary, :primary_preferred, :nearest
        # (default: primary)
          mode: :secondary_preferred
            - use: web

        # The name of the user for authentication.
        user: 'user'

        # The password of the user for authentication.
        password: 'password'

        # The user's database roles.
          - 'dbOwner'

        # Change the default authentication mechanism. Valid options are: :scram,
        # :mongodb_cr, :mongodb_x509, and :plain. (default on 3.0 is :scram, default
        # on 2.4 and 2.6 is :plain)
        auth_mech: :scram

        # The database or source to authenticate the user against. (default: admin)
        auth_source: admin

        # Force the driver to connect in a specific way instead of auto-
        # discovering. Can be one of: :direct, :replica_set, :sharded. Set to :direct
        # when connecting to hidden members of a replica set.
        connect: :direct

        # Change the default time in seconds the server monitors refresh their status
        # via ismaster commands. (default: 10)
        heartbeat_frequency: 10

        # The time in seconds for selecting servers for a near read preference. (default: 0.015)
        local_threshold: 0.015

        # The timeout in seconds for selecting a server for an operation. (default: 30)
        server_selection_timeout: 30

        # The maximum number of connections in the connection pool. (default: 5)
        max_pool_size: 5

        # The minimum number of connections in the connection pool. (default: 1)
        min_pool_size: 1

        # The time to wait, in seconds, in the connection pool for a connection
        # to be checked in before timing out. (default: 1)
        wait_queue_timeout: 1

        # The time to wait to establish a connection before timing out, in seconds.
        # (default: 10)
        connect_timeout: 10

        # The timeout to wait to execute operations on a socket before raising an error.
        # (default: 5)
        socket_timeout: 5

        # The name of the replica set to connect to. Servers provided as seeds that do
        # not belong to this replica set will be ignored.
        replica_set: my_replica_set

        # Whether to connect to the servers via ssl. (default: false)
        ssl: true

        # The certificate file used to identify the connection against MongoDB.
        ssl_cert: /path/to/my.cert

        # The private keyfile used to identify the connection against MongoDB.
        # Note that even if the key is stored in the same file as the certificate,
        # both need to be explicitly specified.
        ssl_key: /path/to/my.key

        # A passphrase for the private key.
        ssl_key_pass_phrase: password

        # Whether or not to do peer certification validation. (default: true)
        ssl_verify: true

        # The file containing a set of concatenated certification authority certifications
        # used to validate certs passed from the other end of the connection.
        ssl_ca_cert: /path/to/ca.cert

  # Configure Mongoid specific options. (optional)
    # Include the root model name in json serialization. (default: false)
    include_root_in_json: false

    # Include the _type field in serialization. (default: false)
    include_type_for_serialization: false

    # Preload all models in development, needed when models use
    # inheritance. (default: false)
    preload_models: false

    # Raise an error when performing a #find and the document is not found.
    # (default: true)
    raise_not_found_error: true

    # Raise an error when defining a scope with the same name as an
    # existing method. (default: false)
    scope_overwrite_exception: false

    # Use Active Support's time zone in conversions. (default: true)
    use_activesupport_time_zone: true

    # Ensure all times are UTC in the app side. (default: false)
    use_utc: false

    # Set the Mongoid and Ruby driver log levels. (default: :info)
    log_level: :info

Inline Configuration

It is also possible to configure Mongoid directly in Ruby, without using a configuration file. This configuration mechanism does not support defining multiple environments, however it does support defining multiple clients.

Mongoid.configure do |config|
  config.clients.default = {
    hosts: ['localhost:27017'],
    database: 'my_db',

  config.log_level = :warn


Mongoid and the MongoDB Ruby driver have separate loggers and log levels.

If Mongoid is loaded in the context of a Rails application, it will use Rails’ configured logger by default. Otherwise Mongoid will configure itself to log to STDOUT with the default log level of INFO.

The Ruby driver’s default log level is DEBUG, and changing Mongoid’s log level does not affect the Ruby driver’s log level. However, if Mongoid is configured with a configuration file, the log level specified in the configuration file will be applied to both the Mongoid and the Ruby driver loggers.

To change the log levels:

# Mongoid
Mongoid.logger.level = Logger::DEBUG
# Ruby driver
Mongo::Logger.logger.level = Logger::INFO

Note: The Mongo client is a Ruby driver client instance, therefore the logger of a Mongo client is the Ruby driver logger, not the Mongoid logger. In other words:

# Ruby driver logger, not Mongoid logger
Mongoid.client(:default).logger == Mongo::Logger.logger

Usage with Forking Servers

When using Mongoid with a forking web server such as Unicorn, worker processes need to recreate driver client instances to ensure that background threads monitoring the MongoDB cluster are recreated as they are not transfered into child processes through forks, and file descriptors and synchronization primitives are not shared between workers.

It is recommended to not perform any operations on Mongoid models in the parent process prior to the fork. If the parent process needs to perform operations on the MongoDB database, reset all clients in an after_fork handler which is defined in unicorn.rb:

after_fork do |server, worker|
  Mongoid.clients.each do |name, client|

If the parent process does not need to perform operations on the MongoDB cluster after child processes are forked, close the client in the parent:

before_fork do |server, worker|
  Mongoid.clients.each do |name, client|

Note: This pattern should be used with Ruby driver version 2.6.2 or higher. Previous driver versions did not recreate monitoring threads when reconnecting.

Note: If the parent process performs operations on the Mongo client and does not close it, the parent process will continue consuming a connection slot in the cluster and will continue monitoring the cluster for as long as the parent remains alive.

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