The belongs_to relation

The belongs_to relation is the child side of the relation. It is set to complement the has_one or has_many relations. This method takes the following options in addition to :autosave, :autobuild, :dependent, :primary_key, and :foreign_key.

:index

The index option determines if the foreign key is indexed or not. It's recommended that foreign keys must be indexed. The values are set to true or false as shown in the following code:

class Book
  include Mongoid::Document

  belongs_to :author, index: true
end

:polymorphic

This is used to complement the :as option for the parent relationship. This option sets the polymorphic resource as follows:

class Vehicle
  include Mongoid::Document

  belongs_to :resource, polymorphic: true
end

:touch

The touch method allows the child object to update the updated_at field of the parent when the child is changed.

:counter_cache

When we add or remove a child, we often require the parent object to be updated with a counter. This increment or decrement...