docs: def _alias_id(self, primary_id, equivalent_id):
          """Adds the given equivalent_id as an alias for primary_id if possible"""
          pkg_name = primary_id.get_identifier_namespace().split('.')[0]
          obj_name = primary_id.get_identifier_namespace().split('.')[1]
          collection = JSONClientValidated(pkg_name,
                                           collection=obj_name,
                                           runtime=self._runtime)
          collection.find_one({'_id': ObjectId(primary_id.get_identifier())})  # to raise NotFound
          collection = JSONClientValidated('id',
                                           collection=pkg_name + 'Ids',
                                           runtime=self._runtime)
          try:
              result = collection.find_one({'aliasIds': {'$in': [str(equivalent_id)]}})
          except errors.NotFound:
              pass
          else:
              result['aliasIds'].remove(str(equivalent_id))
              collection.save(result)
          try:
              id_map = collection.find_one({'_id': str(primary_id)})
          except errors.NotFound:
              collection.insert_one({'_id': str(primary_id), 'aliasIds': [str(equivalent_id)]})
          else:
              id_map['aliasIds'].append(str(equivalent_id))
              collection.save(id_map)