code: function normalizeEntity(entity) {
      if (entity !== null &&
              typeof entity !== 'undefined' &&
              typeof entity.getId === 'function') {
          return entity.getId();
      }
      return entity;
  }
docs: Normalizes `entity` to an id, where `entity` can be an id
  or a Model instance.
  
  @param  {*} entity - either a Model instance or an id value
  @return {*} the id value of `entity`