code: def get_repository_ids_by_composition(self, composition_id):
          
          mgr = self._get_provider_manager('REPOSITORY', local=True)
          lookup_session = mgr.get_composition_lookup_session(proxy=self._proxy)
          lookup_session.use_federated_repository_view()
          lookup_session.use_unsequestered_composition_view()
          composition = lookup_session.get_composition(composition_id)
          id_list = []
          if 'assignedRepositoryIds' in composition._my_map:
              for idstr in composition._my_map['assignedRepositoryIds']:
                  id_list.append(Id(idstr))
          return IdList(id_list)
docs: Gets the ``Repository``  ``Ids`` mapped to a ``Composition``.
  
          arg:    composition_id (osid.id.Id): ``Id`` of a ``Composition``
          return: (osid.id.IdList) - list of repository ``Ids``
          raise:  NotFound - ``composition_id`` is not found
          raise:  NullArgument - ``composition_id`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*