collect_join_reflections

# File activerecord/lib/active_record/reflection.rb, line 1000
      def collect_join_reflections(seed)
        a = source_reflection.add_as_source seed
        if options[:source_type]
          through_reflection.add_as_polymorphic_through self, a
        else
          through_reflection.add_as_through a
        end
      end