Rails Pocket Reference

Many-to-many

There are two ways to build a many-to-many relationship. The first and preferred way uses a has_many association with the :through option and a join model, so there are two stages of associations:

class UserRole < ActiveRecord::Base
  belongs_to :user
  belongs_to :role
end

class User < ActiveRecord::Base
  has_many :user_roles
  has_many :roles, :through => :user_roles
end

class Role < ActiveRecord::Base
  has_many :user_roles
  has_many :users, :through => :user_roles
end

The second way uses has_and_belongs_to_many in both models. This requires a join table that has no corresponding model or primary key:

class User < ActiveRecord::Base
  has_and_belongs_to_many :roles
end

class Role < ActiveRecord::Base
  has_and_belongs_to_many :users
end

See Figure 3.

Figure 3. Many-to-many associations