code: def _get_entity_ids(field_name, attrs):
      
      field_name_ids = field_name + '_ids'
      plural_field_name = pluralize(field_name)
      if field_name_ids in attrs:
          return attrs[field_name_ids]
      elif field_name in attrs:
          return [entity['id'] for entity in attrs[field_name]]
      elif plural_field_name in attrs:
          return [entity['id'] for entity in attrs[plural_field_name]]
      else:
          raise MissingValueError(
              'Cannot find a value for the "{0}" field. Searched for keys named '
              '{1}, but available keys are {2}.'
              .format(
                  field_name,
                  (field_name_ids, field_name, plural_field_name),
                  attrs.keys()
              )
          )
docs: Find the IDs for a one to many relationship.
  
      The server may return JSON data in the following forms for a
      :class:`nailgun.entity_fields.OneToManyField`::
  
          'user': [{'id': 1, …}, {'id': 42, …}]
          'users': [{'id': 1, …}, {'id': 42, …}]
          'user_ids': [1, 42]
  
      Search ``attrs`` for a one to many ``field_name`` and return its ID.
  
      :param field_name: A string. The name of a field.
      :param attrs: A dict. A JSON payload as returned from a server.
      :returns: An iterable of entity IDs.