has_many_tp2.rb

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

class Project < ActiveRecord::Base
  has_many :memberships, as: :memberable, dependent: :destroy
  has_many :users, through: :memberships
end

class Group < ActiveRecord::Base
  has_many :memberships, as: :memberable, dependent: :destroy
  has_many :users, through: :memberships
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