has_one – the one-to-one relation

As the method name suggests, has_one sets up the parent relation for a model having only one child:

class Book
  include Mongoid::Document

  has_one :book_detail
end

This implies that "A Book class has one BookDetail". For every parent relation, it is advisable to set the child relationship too.

class BookDetail
  include Mongoid::Document

  belongs_to :book
end

The has_one method takes the options discussed in the following sections.

:as

When a relation is a polymorphic relation, we need to use this option:

class Ship
  include Mongoid::Document  

  has_one :vehicle, as: resource
end

This tells the has_one method that the vehicle is a polymorphic relation that can be accessed via the resource_type and resource_id fields in the vehicles collection. It's important that the inverse relation sets polymorphic: true, and we will see polymorphic relations soon.

:autosave

The :autosave option is false by default. When the object is created, the related child objects...