has_and_belongs_to_many – the many-to-many relation

Let's assume that books belong to many categories and categories have many books. This is the Many-to-Many relationship method. A typical class would look like the following:

class Book
  include Mongoid::Document

  has_and_belongs_to_many :categories
end

class Category
  include Mongoid::Document

  has_and_belongs_to_many :books
end

It takes all the standard options such as :autosave, :dependent, :foreign_key, :index, :primary_key, and :order. It also supports the :before_add, :after_add, :before_remove, and :after_remove relation callbacks.

Note

A Many-to-Many relation cannot be a part of a polymorphic relation. This is because a polymorphic relation expects an explicit parent-child relationship and Many-to-Many relations are peer relations.

:inverse_of

Among all the other options, the inverse_of relation is a very interesting one. As with Many-to-Many relations, the document IDs are stored as arrays on both sides of the association. So...