valid? (DataMapper::Associations::Relationship)

    # File lib/dm-core/associations/relationship.rb, line 334
          def valid?(source)
            return true if source.nil?

            case source
              when Array, Collection then valid_collection?(source)
              when Resource          then valid_resource?(source)
              else
                raise ArgumentError, "+source+ should be an Array or Resource, but was a #{source.class.name}"
            end
          end