MONGOID

You can define indexes on documents using the index macro. For unique indexes provide a unique options, otherwise no option is necessary.

class Person
  include Mongoid::Document
  field :ssn
  index :ssn, :unique => true
end

You can define indexes on embedded document fields as well.

class Person
  include Mongoid::Document
  embeds_many :addresses
  index "addresses.street"
end

You can index on multiple fields and provide direction.

class Person
  include Mongoid::Document
  field :first_name
  field :last_name
  index(
    [
      [ :first_name, Mongo::ASCENDING ],
      [ :last_name, Mongo::ASCENDING ]
    ],
    :unique => true
  )
end

Indexes can be run in the background in cases where they may take some time:

class Person
  include Mongoid::Document
  field :ssn
  index :ssn, :unique => true, :background => true
end

For geospacial indexes, make sure the field you are indexing is an Array.

class Person
  include Mongoid::Document
  field :location, :type => Array
  index [[ :location, Mongo::GEO2D ]], :min => 200, :max => 200
end

You can have Mongoid define indexes for you on "foreign key" fields for relational associations.

class Comment
  include Mongoid::Document
  referenced_in :post, :inverse_of => :comments, :index => true
  references_many \
    :users,
    :stored_as => :array,
    :inverse_of => :comments,
    :index => true
end

When you want to create the indexes in the database, use the provided rake task:

rake db:create_indexes

If you want indexes autocreated when the model classes are loaded, add this configuration option to your mongoid.yml. Note this is NOT recommended for any production environment.

defaults: &defaults
  autocreate_indexes: true