# File lib/dm-core/associations/relationship.rb, line 335
      def valid?(value, negated = false)
        case value
          when Enumerable then valid_target_collection?(value, negated)
          when Resource   then valid_target?(value)
          when nil        then true
          else
            raise ArgumentError, "+value+ should be an Enumerable, Resource or nil, but was a #{value.class.name}"
        end
      end