The code for has_many :through polymorphic is pretty straightforward but what if you are using Single Table Inheritance (STI) on either or both sides of the polymorphic through table?

There were a number of painful hacks to make this work until a patch to ActiveRecord by Trevor Squires added the :source_type key to has_many :through polymorphic (hmtp). Check it out here: [PATCH] allow polymorphic :source for has_many :through

The main purpose of this blog post is to write up an example of using :source_type in the hope that people won’t get tripped up by out-of-date blog posts which delt with hmtp before the :source_type patch.

The Scenario:

Your web application has Users and Organizations and users belong to an organization through Memberships.
This is a typical example of where you might use has_many :through. But what if you have many subclasses of users - like Student and Admin - and/or you have subclasses of Organizations - like StudentOrganization and AthleticTeam. To make this work you’ll need the :source_type key. Once you have that it’s quite simple:

The Key Models:

class User < ActiveRecord::Base
has_many :memberships
has_many :organizations, :through => :memberships, :source => :member_of, :source_type => 'Organization'
end

class Membership < ActiveRecord::Base
belongs_to :user
belongs_to :member_of, :polymorphic => true
end

class Organization < ActiveRecord::Base
has_many :memberships, :as => :member_of
has_many :users, :through => :memberships
end

The Other Models:

class Student < User
end
class Admin < User
end

class StudentOrganization < Organization
end

class AthleticTeam < Organization
end