docs: def get_relationships_by_ids(self, relationship_ids=None):
          """Gets a ``RelationshipList`` corresponding to the given ``IdList``.
  
          arg:    relationship_ids (osid.id.IdList): the list of ``Ids``
                  to retrieve
          return: (osid.relationship.RelationshipList) - the returned
                  ``Relationship list``
          raise:  NotFound - an ``Id`` was not found
          raise:  NullArgument - ``relationship_ids`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*
  
          """
          if relationship_ids is None:
              raise NullArgument()
          relationships = []
          for i in relationship_ids:
              relationship = None
              url_path = ('/handcar/services/relationship/families/' +
                          self._catalog_idstr +
                          '/relatioships/' + str(i))
              try:
                  relationship = self._get_request(url_path)
              except (NotFound, OperationFailed):
                  if self._relationship_view == PLENARY:
                      raise
                  else:
                      pass
              if relationship:
                  if not (self._relationship_view == COMPARATIVE and
                          relationship in relationships):
                      relationships.append(relationship)
          return objects.RelationshipList(relationships)