def references(attributes=nil)
      attributes ||= self.class.domain_attributes
      attributes.map { |pa| send(pa) }.flatten.compact
    end
def references(attributes=nil)
      attributes ||= self.class.domain_attributes
      attributes.map { |pa| send(pa) }.flatten.compact
    end
Returns the domain object references for the given attributes.

 @param [<Symbol>, nil] the domain attributes to include, or nil to include all domain attributes
 @return [<Resource>] the referenced attribute domain object values