source_reflection_name

# File activerecord/lib/active_record/reflection.rb, line 1115
      def source_reflection_name # :nodoc:
        @source_reflection_name ||= begin
          names = [name.to_s.singularize, name].collect(&:to_sym).uniq
          names = names.find_all { |n|
            through_reflection.klass._reflect_on_association(n)
          }

          if names.length > 1
            raise AmbiguousSourceReflectionForThroughAssociation.new(
              active_record.name,
              macro,
              name,
              options,
              source_reflection_names
            )
          end
          names.first
        end
      end