collect_join_reflections

# File activerecord/lib/active_record/reflection.rb, line 978
        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