docs: def make_relationship(self, relator,
                            direction=
                              RELATIONSHIP_DIRECTIONS.BIDIRECTIONAL):
          """
          Create a relationship object for this attribute from the given
          relator and relationship direction.
          """
          if IEntity.providedBy(relator): # pylint:disable=E1101
              rel = DomainRelationship(relator, self,
                                       direction=direction)
          elif IResource.providedBy(relator): # pylint:disable=E1101
              rel = ResourceRelationship(relator, self,
                                         direction=direction)
          else:
              raise ValueError('Invalid relator argument "%s" for '
                               'relationship; must provide IEntity or '
                               'IResource.' % relator)
          return rel