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)
        # These options are Ruby driver options, documented in

        # 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: nil)
        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

        # Compressors to use. (default is to not use compression)
        compressors: [zlib]

  # 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 ActiveSupport's time zone in time operations instead of
    # the Ruby default time zone. See the time zone section below for
    # further information. (default: true)
    use_activesupport_time_zone: true

    # Return stored times as UTC. See the time zone section below for
    # further information. Most applications should not use this option.
    # (default: false)
    use_utc: false

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

    # Whether to join nested persistence contexts for atomic operations
    # to parent contexts by default. (default: false)
    join_contexts: false

The Ruby driver options may be found in the driver documentation.

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

Time Zones

Ruby has limited time zone support in the standard library. ActiveSupport (which Mongoid depends on) offers more comprehensive time zone support. Importantly, Ruby and ActiveSupport may be configured with different default time zones.

While a thorough treatment of time zones in Ruby is outside the scope of this tutorial, the easiest and most reliable way of achieving correct time zone handling is as follows:

  1. Set the operating system’s time zone to UTC. For example, on Linux:
cp /usr/share/zoneinfo/UTC /etc/localtime
  1. Set ActiveSupport’s time zone to UTC:
# If using Rails, in application.rb:
class Application < Rails::Application
  config.time_zone = 'UTC'

# If not using Rails: = 'UTC'
  1. Store and persist all times in UTC. Perform all calculations on times in UTC.
  2. When working with user input in local time, convert such user input to UTC times as soon as possible, and then work with the UTC times.
  3. When rendering or otherwise presenting times, convert them to local time after performing all calculations, when actually rendering.
  4. Date to time (for example, the time when a particular day starts or ends) conversions are a common source of errors. Such conversions should generally be performed while explicitly specifying the time zone in which the date is understood to be.

Applications using Mongoid should generally configure ActiveSupport’s time zone as described above, and then use rather than Time (for example, instead of to invoke the ActiveSupport time zone machinery. This also helps achieve correct results when the system time zone is not UTC, as is common in development environments.

Note that MongoDB stores all times in UTC without time zone information.

Mongoid offers the following time zone-related configuration options:

  • use_activesupport_time_zone: If true, prefer to work with times using ActiveSupport::TimeWithZone. Values in fields of type Time will be returned as instances of ActiveSupport::TimeWithZone. When parsing times without time zone information (such as when mongoizing strings or arrays to time), assume the times are specified in ActiveSupport’s time zone. This is the default.

    If false, prefer to work with times using Ruby standard library Time class. Values in fields of type Time will be returned as Time instances. When parsing times without time zone information, assume the times are specified in the Ruby time zone.

    Note that the use_activesupport_time_zone setting does not affect fields of types Date or DateTime, which use Date and DateTime classes for their values, respectively.

    Also note that Mongoid may still utilize both Time and ActiveSupport::TimeWithZone classes internally, as appropriate, regardless of the use_activesupport_time_zone setting.

  • use_utc: If true, times stored in MongoDB will be returned in UTC. If false, times stored in MongoDB will be returned in local time (as instances of either Time or ActiveSupport::TimeWithZone, respectively in the Ruby default time zone or the ActiveSupport time zone, based on the value of use_activesupport_time_zone setting). The default is false.

    The use_utc setting does not affect how times are parsed - parsing is always done in local time when the input being parsed does not include time zone information. To parse dates in UTC, set the system/Ruby or ActiveSupport time zone to UTC (as mentioned above, setting all three to UTC leads to the fewest headaches).

    Setting use_activesupport_time_zone to true and to UTC (and using ActiveSupport time machinery for all time-related operations) is recommended over setting use_utc to true.

Note that use_activesupport_time_zone and use_utc options do not throw away time zone information when it is available. For example, a Time instance does have an associated time zone, and this time zone will be used even if it is different from ActiveSupport’s configured time zone when use_activesupport_time_zone is true.

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.

Development Configuration

Driver’s default configuration is suitable for production deployment. In development, some settings can be adjusted to provide a better developer experience.

  • :server_selection_timeout: set this to a low value (e.g., 1)

if your MongoDB server is running locally and you start it manually. A low server selection timeout will cause the driver to fail quickly when there is no server running.

Sample recomended development configuration:

      database: mongoid
        - localhost:27017
        server_selection_timeout: 1
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