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.

Generating Default Configuration

If you are using Ruby on 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.

If you are not using Ruby on Rails, you can copy the minimal configuration given above and save it as config/mongoid.yml.

Loading Mongoid Configuration

If you are using Ruby on Rails, Mongoid configuration is automatically loaded for the current environment as stored in Rails.env when the application loads.

You may need to configure the ORM for your application to be Mongoid by adding the following to application.rb:

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

If you are not using Ruby on Rails, Mongoid configuration must be loaded manually. This can be done via the Mongoid.load! method, which takes the configuration file path as its argument, as follows:

# Use automatically detected environment name

# Specify environment name manually
Mongoid.load!("path/to/your/mongoid.yml", :production)

When Mongoid is asked to automatically detect the environment name, it does so by examining the following sources, in order:

  • If Rails top level constant is defined, Rails.env.
  • If Sinatra top level constant is defined, Sinatra::Base.environment.
  • The RACK_ENV environment variable.
  • The MONGOID_ENV environment variable.

It is also possible to configure Mongoid directly in Ruby, without using a configuration file. This configuration style does not support the concept of environments - whatever configuration is provided, it is applied to the current environment - but it does support defining multiple clients.

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

  config.log_level = :warn


Mongoid must be configured before any component of it is used or referenced. Once a component is used or referenced, changing configuration may not apply changes to already instantiated components.

Mongoid Configuration Options

The following annotated example mongoid.yml demonstrates how Mongoid can be configured.

Mongoid delegates to the Ruby driver for client configuration. Please review the driver documentation for details on driver options.

  # 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. Please see the
        # driver documentation linked above for details on how to configure
        # authentication. Valid options are :aws, :gssapi, :mongodb_cr,
        # :mongodb_x509, :plain, :scram and :scram256 (default on 3.0
        # and higher servers is :scram, default on 2.6 servers is :plain)
        auth_mech: :scram

        # Specify the auth source, i.e. the database or other source which
        # contains the user's login credentials. Allowed values for auth source
        # depend on the authentication mechanism, as explained in the server documentation:
        # If no auth source is specified, the default auth source as
        # determined by the driver will be used. Please refer to:
        auth_source: admin

        # Connect directly to and perform all operations on the specified
        # server, bypassing replica set node discovery and monitoring.
        # Exactly one host address must be specified. (default: false)
        #direct_connection: true

        # Deprecated. Force the driver to connect in a specific way instead
        # of automatically discovering the deployment type and connecting
        # accordingly. To connect directly to a replica set node bypassing
        # node discovery and monitoring, use direct_connection: true instead
        # of this option. Possible values: :direct, :replica_set, :sharded.
        # (default: none)
        #connect: :direct

        # Change the default time in seconds the server monitors refresh their status
        # via hello 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

        # How long to wait for a response for each operation sent to the
        # server. This timeout should be set to a value larger than the
        # processing time for the longest operation that will be executed
        # by the application. Note that this is a client-side timeout;
        # the server may continue executing an operation after the client
        # aborts it with the SocketTimeout exception.
        # (default: nil, meaning no timeout)
        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)
    # Application name that is printed to the MongoDB logs upon establishing
    # a connection in server versions 3.4 or greater. Note that the name
    # cannot exceed 128 bytes in length. It is also used as the database name
    # if the database name is not explicitly defined. (default: nil)
    app_name: MyApplicationName

    # Mark belongs_to associations as required by default, so that saving a
    # model with a missing belongs_to association will trigger a validation
    # error. (default: true)
    belongs_to_required_by_default: true

    # Maintain broken behavior of sum over empty result sets for backwards
    # compatibility. When calculating a sum on a field with a null context,
    # for example:
    # Product.none.sum(:price)
    # ... return field name (`:price') instead of 0.
    # When calculating a sum via a database query with an empty result set,
    # for example:
    # Product.where(impossible_condition: true).sum(:price)
    # ... return nil instead of 0.
    # (default: false)
    #broken_aggregables: true

    # Ignore aliased fields in embedded documents when performing pluck and
    # distinct operations, for backwards compatibility.
    # (default: false)
    #broken_alias_handling: true

    # Maintain broken `and' method behavior that existed in Mongoid 7.3
    # and earlier for backwards compatibility: in some situations, conditions
    # already present in a Criteria object would be replaced by newer
    # conditions when `and' method is used instead of the new conditions
    # being added to the existing conditions. This would happen when using
    # the same operator on the same field multiple times. For example:
    # Band.where(id: 1).and({year: {'$in' => [2020]}}, {year: {'$in' => [2021]}}).where(id: 2)
    # yields the following criteria:
    # <Mongoid::Criteria
    # selector: {"_id"=>1, "year"=>{"$in"=>[2020]}, "$and"=>[{"_id"=>2}]}
    # options:  {}
    # class:    Band
    # embedded: false>
    # This is obviously incorrect as the {"$in"=>[2021]} clause is lost.
    # Notice that the clause is only lost when both clauses are added using
    # the #and method.
    # (default: false)
    #broken_and: true

    # When exiting a nested `with_scope' block, set the current scope to
    # nil instead of the parent scope for backwards compatibility.
    # (default: false)
    #broken_scoping: true

    # Maintain broken update behavior in some cases for backwards
    # compatibility.
    # In Mongoid 7.3 and earlier, when assigning a value to an embedded
    # document, then setting it to nil, then assigning the original value
    # to it again, the second update would not work and the value for the
    # embedded document would remain nil. Take this case:
    # canvas.palette = palette
    # canvas.palette = nil
    # canvas.palette = palette
    # ... where canvas embeds_one palette.
    # In Mongoid 7.3 and earlier, canvas.palette would be nil when we would
    # expect it to be palette. Set this option to true to keep this behavior,
    # set the option to false to perform the second update correctly.
    # (default: false)
    #broken_updates: true

    # Time objects in Ruby have nanosecond precision, whereas MongoDB server
    # can only store times with millisecond precision. Set this option to
    # true to truncate times to millisecond precision when performing
    # queries on already loaded embedded associations (this is also called
    # "embedded matching" and is done completely in Ruby), to obtain the
    # same query results when performing time comparisons regardless of
    # which documents are being queried. Setting this option to false will
    # produce different results for queries on embedded associations that
    # are already loaded into memory vs queries on unloaded associations and
    # top-level models. (default: true)
    #compare_time_by_ms: false

    # Set the global discriminator key. (default: "_type")
    discriminator_key: "_type"

    # Raise an exception when a field is redefined. (default: false)
    duplicate_fields_exception: false

    # 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

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

    # When this flag is true, the attributes method on a document will return
    # a BSON::Document when that document is retrieved from the database, and
    # a Hash otherwise. When this flag is false, the attributes method will
    # always return a Hash. (default: false)
    #legacy_attributes: true

    # Maintain legacy behavior of pluck and distinct, which does not demongoize
    # values on returning them. Setting this option to false will cause
    # pluck and distinct to return demongoized values. Setting this option to
    # false will also allow retrieving *_translations fields from pluck and
    # distinct and will return embedded values themselves (i.e. without
    # putting them in a hash).
    # (default: false)
    #legacy_pluck_distinct: true

    # Maintain legacy behavior of === on Mongoid document classes, which
    # returns true in a number of cases where Ruby's === implementation would
    # return false. Note that the behavior of === on Mongoid document
    # instances differs from both the behavior of === on document classes
    # and from Ruby's behavior of === on simple object instances regardless
    # of the value of this option.
    # (default: false)
    #legacy_triple_equals: true

    # Set the Mongoid and Ruby driver log levels when Mongoid is not using
    # Ruby on Rails logger instance. (default: :info)
    log_level: :info

    # When using the BigDecimal field type, store the value in the database
    # as a BSON::Decimal128 instead of a string. (default: true)
    #map_big_decimal_to_decimal128: true

    # Force ``BSON::ObjectId#as_json`` method to return the hash
    # { "$oid" => id.to_s }. When this option is false, and bson-ruby 5
    # is used, the return value will be the hexadecimal ObjectId string only.
    # (default: false)
    #object_id_as_json_oid: true

    # When chaining the same operators that use the same field, setting this
    # feature flag to false will cause those operators to be combined using an
    # and. Setting this feature flag to true will cause the later chained
    # operators to overwrite the earlier ones. (default: false)
    #overwrite_chained_operators: 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

    # (Deprecated) In MongoDB 4.0 and earlier, set whether to create
    # indexes in the background by default. (default: false)
    background_indexing: false
  # Configure driver-specific options. (optional)
    # When this flag is turned off, inline options will be correctly
    # propagated to Mongoid and Driver finder methods. When this flag is turned
    # on those options will be ignored. For example, with this flag turned
    # off, Band.all.limit(1).count will take the limit into account, while
    # when this flag is turned on, that limit is ignored. The affected driver
    # methods are: aggregate, count, count_documents, distinct, and
    # estimated_document_count. The corresponding Mongoid methods are also
    # affected. (default: false, driver version: 2.18.0+)
    #broken_view_options: false

    # Validates that there are no atomic operators (those that start with $)
    # in the root of a replacement document, and that there are only atomic
    # operators at the root of an update document. If this feature flag is on,
    # an error will be raised on an invalid update or replacement document,
    # if not, a warning will be output to the logs. This flag does not affect
    # Mongoid as of 8.0, but will affect calls to driver update/replace
    # methods. (default: false, driver version: 2.18.0+)
    #validate_update_replace: false

ERb Preprocessing

When loading a configuration file, Mongoid processes it with ERb before parsing it as YAML. This allows, for example, constructing the contents of the configuration file at runtime based on environment variables:

      uri: "<%= ENV['MONGODB_URI'] %>"


When outputting values from ERb, ensure the values are valid YAML and escape them as needed.


Since ERb rendering is performed prior to YAML parsing, all ERb directives in the configuration file are evaluated, including those occurring in YAML comments.


When configuring logging, it is important to keep in mind that Mongoid provides a model layer on top of the MongoDB Ruby driver, and the driver dispatches the CRUD operations to the MongoDB deployment. Therefore, some of the logging output in an application using Mongoid comes from Mongoid itself, and some comes from the driver.

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

Depending on whether Mongoid is used in a Ruby on Rails application, and how both Mongoid and Ruby driver are configured, they may use the same logger instance or different instances, potentially with different configurations.

In Ruby on Rails Application

When used in a Ruby on Rails application, Mongoid by default inherits the logger and the log level from Rails, and sets the driver’s logger to the same logger instance:

Rails.logger === Mongoid.logger
# => true

Mongoid.logger === Mongo::Logger.logger
# => true

To change the log level, use standard Rails configuration. Place the following in one of environment configuration files, such as config/environments/production.rb:

Rails.application.configure do
  config.log_level = :debug


The log_level Mongoid configuration option is not used when Mongoid operates in a Rails application, because Mongoid inherits Rails’ log level in this case.

To configure either Mongoid or driver logger differently from the Rails logger, use an initializer as follows:

Rails.application.configure do
  config.after_initialize do
    # Change Mongoid log destination and/or level
    Mongoid.logger = do |logger|
      logger.level = Logger::DEBUG

    # Change driver log destination and/or level
    Mongo::Logger.logger = do |logger|
      logger.level = Logger::DEBUG


There is currently no provision in the Ruby standard library Logger to return the log device (i.e. the IO object) that a logger is using. To have, for example, Mongoid and/or the Ruby driver log to the standard Rails log file (e.g. log/development.log) but with a different level from standard Rails logger (Rails.logger), the file must be opened separately and the resulting IO object passed to the Logger constructor.


Since by default Mongoid sets its own logger and the driver’s logger to the same instance as the Rails logger, modifying any of the instances affects all of them. For example the following changes log level for all three loggers, unless the application assigned a separate Logger instance to Mongo::Logger.logger` as described above:

Mongoid::Logger.logger.level = Logger::DEBUG


When not loaded in a Ruby on Rails application, Mongoid respects the log_level top level configuration option. It can be given in the configuration file as follows:

      # ...
    log_level: :debug

… or when configuring Mongoid inline:

Mongoid.configure do |config|
  config.log_level = :debug

The default log destination in Mongoid 7.1 and higher is standard error. The default log destination in Mongoid 7.0 and lower is standard output. To change the log destination, create a new logger instance as follows:

Mongoid.logger = do |logger|
  logger.level = Logger::DEBUG

To change the Ruby driver log level or destination:

Mongo::Logger.logger = do |logger|
  logger.level = Logger::DEBUG

To set the driver logger to be the same as the Mongoid logger:

Mongo::Logger.logger = Mongoid.logger


Mongoid does not alter the driver’s logger when running in standalone mode.

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.

Configuring SSLContext

It may be desirable to further configure TLS options in your application, for example by enabling or disabling certain ciphers.

This can be done by setting TLS context hooks on the Ruby driver – TLS context hooks are user-provided Proc``s that will be invoked before any TLS socket connection in the driver and can be used to modify the underlying ``OpenSSL::SSL::SSLContext object used by the socket.

To set TLS context hooks, add Proc``s to the ``Mongo.tls_context_hooks array. This can be done in an initializer. The example below adds a hook that only enables the “AES256-SHA” cipher.

Mongo.tls_context_hooks.push( { |context|
    context.ciphers = ["AES256-SHA"]

# Only the AES256-SHA cipher will be enabled from this point forward

Every Proc in Mongo.tls_context_hooks will be passed an OpenSSL::SSL::SSLContext object as its sole argument. These procs will be executed sequentially during socket creation.


TLS context hooks are global and will affect all ``Mongo::Client`` instances
in an application.

For more information about TLS context hooks, including best practices for assigning and removing them, see the Ruby driver documentation.

Client-Side Encryption

When loading the configuration file, Mongoid permits the file to contain BSON::Binary instances which are used for specifying keyId in the schema map for client-side encryption, as the following example shows:

      database: blog_development
      hosts: [localhost:27017]
          key_vault_namespace: 'keyvault.datakeys'
              key: "z7iYiYKLuYymEWtk4kfny1ESBwwFdA58qMqff96A8ghiOcIK75lJGPUIocku8LOFjQuEgeIP4xlln3s7r93FV9J5sAE7zg8U"
                      - !ruby/object:BSON::Binary
                        data: !binary |-
                        type: :uuid
                    bsonType: "string"
                    algorithm: "AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic"
              bsonType: "object"

Usage with Forking Servers

When using Mongoid with a forking web server such as Puma, Unicorn or Passenger, it is recommended to not perform any operations on Mongoid models in the parent process prior to the fork.

When a process forks, Ruby threads are not transferred to the child processes and the Ruby driver Client objects lose their background monitoring. The application will typically seem to work just fine until the deployment state changes (for example due to network errors, a maintenance event) at which point the application is likely to start getting NoServerAvailable exception when performing MongoDB operations.

If the parent process needs to perform operations on the MongoDB database, reset all clients in the workers after they forked. How to do so depends on the web server being used.

If the parent process does not need to perform operations on the MongoDB database after child processes are forked, close the clients in the parent prior to forking children. If the parent process performs operations on a 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.


The close/reconnect pattern described here should be used with Ruby driver version 2.6.2 or higher. Previous driver versions did not recreate monitoring threads when reconnecting.


Use the on_worker_boot hook to reconnect clients in the workers and the before_fork hook to close clients in the parent process (Puma documentation):

on_worker_boot do
  Mongoid::Clients.clients.each do |name, client|

before_fork do


Use the after_fork hook to reconnect clients in the workers and the before_fork hook to close clients in the parent process (Unicorn documentation):

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

before_fork do |server, worker|


Use the starting_worker_process hook to reconnect clients in the workers (Passenger documentation). Passenger does not appear to have a hook that is invoked in the parent process before the workers are forked.

if defined?(PhusionPassenger)
  PhusionPassenger.on_event(:starting_worker_process) do |forked|
    Mongoid::Clients.clients.each do |name, client|

Query Cache Middleware


When used with Ruby driver version 2.15 or newer, Mongoid’s Query Cache Middleware delegates to the driver’s Query Cache Middleware.

Mongoid provides a Rack middleware which enables the Query Cache for the duration of each web request. Below is an example of how to enable the Query Cache Middleware in a Ruby on Rails application:

# config/application.rb

# Add Mongoid::QueryCache::Middleware at the bottom of the middleware stack
# or before other middleware that queries MongoDB.
config.middleware.use Mongoid::QueryCache::Middleware

Please refer to the Rails on Rack guide for more information about using Rack middleware in Rails applications.

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 recommended development configuration:

      database: mongoid
        - localhost:27017
        server_selection_timeout: 1