code: func (user *User) UpdateCache(key string, update interface{}, res interface{}) error {
  
  	exists := user.ctx.updateCacheVal("user", key, update, res)
  
  	if !exists {
  		log.WithField("key", key).Error("Can't update user cache value")
  	}
  	return nil
  }
docs: // UpdateCache updates the per User cache using MongoDB Update query