code: def get_relationship(self, relationship_id=None):
          
          if relationship_id is None:
              raise NullArgument()
          url_path = ('/handcar/services/relationship/families/' +
                      self._catalog_idstr +
                      '/relationships/' + str(relationship_id))
          return objects.Relationship(self._get_request(url_path))
docs: Gets the ``Relationship`` specified by its ``Id``.
  
          arg:    relationship_id (osid.id.Id): the ``Id`` of the
                  ``Relationship`` to retrieve
          return: (osid.relationship.Relationship) - the returned
                  ``Relationship``
          raise:  NotFound - no ``Relationship`` found with the given
                  ``Id``
          raise:  NullArgument - ``relationship_id`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*