Create a many-to-many association for more of 2 classes through a relationship class

I am using Ruby on Rails 3 and I would like to set a many-to-many association for more of 2 classes through a relationship class.

For example, I have:

class RelationshipGroup < ActiveRecord::Base # This is the relationship class

  # Maybe the Schema Information should be the following:
  #
  # Table name: RelationshipGroup
  #
  # id    :开发者_StackOverflow中文版 integer
  # dog_id: integer
  # cat_id: integer
  # cow_id: integer

  ...
end 

class Dog < ActiveRecord::Base
  ...
end 

class Cat < ActiveRecord::Base
  ...
end 

class Cow < ActiveRecord::Base
  ...
end 

In the above example I would like to set those associations in order to can make a search using the RelationshipGroup class and so that I can retrieve all animals belonging to a group. I know how to use the has_many :through association for 2 classes, but not for more of 2. So, it is possible to make that (maybe I have to use an Association Extension or a Class method to reach that?)? If so, how I must write the code for the above example?

---

You could use a polymorphic assocation through a join table.

class RelationshipGroup < ActiveRecord::Base # This is the relationship class
  has_many :memberships
  has_many :members, :through => :memberships
end 

class Membership
  #fields - relationship_group_id, member_id, member_type
  belongs_to :relationship_group
  belongs_to :member, :polymorphic => true
end

class Dog < ActiveRecord::Base
  has_many :memberships, :as => :member
  has_many :relationship_groups, :through => :memberships
end 

class Cat < ActiveRecord::Base
  has_many :memberships, :as => :member
  has_many :relationship_groups, :through => :memberships    
end 

class Cow < ActiveRecord::Base
  has_many :memberships, :as => :member
  has_many :relationship_groups, :through => :memberships    
end 

To go a bit further, it would be good to DRY this up by moving the associations (which are all identical) into a module:

#in lib/member.rb
module Member
  def self.included(base)
    base.class_eval do 
      has_many :memberships, :as => :member
      has_many :relationship_groups, :through => :memberships   
    end    
  end     
end 

class RelationshipGroup < ActiveRecord::Base # This is the relationship class
  has_many :memberships
  has_many :members, :through => :memberships
end 

class Membership
  #fields - relationship_group_id, member_id, member_type
  belongs_to :relationship_group
  belongs_to :member, :polymorphic => true
end

class Dog < ActiveRecord::Base
  include Member
end 

class Cat < ActiveRecord::Base
  include Member 
end 

class Cow < ActiveRecord::Base
  include Member
end