has_many_tp3.rb

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :memberable, polymorphic: true
end

module Memberable
  extend ActiveSupport::Concern
  included do
    has_many :memberships, as: :memberable, dependent: :destroy
    has_many :users, through: :memberships
    after_create :create_membership
  end

  def create_membership
    Membership.create(memberable_type: self.class.name, memberable_id: self.id, user_id: self.creator_id)
  end
end

class Project < ActiveRecord::Base
  include Memberable
end

class Group < ActiveRecord::Base
  include Memberable
end

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, through: :memberships, source: :memberable, source_type: 'Group'
  has_many :projects, through: :memberships, source: :memberable, source_type: 'Project'
end