docs: def get_relationship(self, attribute):
          """
          Returns the domain relationship object for the given resource
          attribute.
          """
          rel = self.__relationships.get(attribute.entity_attr)
          if rel is None:
              rel = LazyDomainRelationship(self, attribute,
                                           direction=
                                              self.relationship_direction)
              self.__relationships[attribute.entity_attr] = rel
          return rel