code: func newModelExistsHandler(collection *Collection, modelID string) ReplyHandler {
  	return func(reply interface{}) error {
  		exists, err := redis.Bool(reply, nil)
  		if err != nil {
  			return err
  		}
  		if !exists {
  			return ModelNotFoundError{
  				Collection: collection,
  				Msg:        fmt.Sprintf("Could not find %s with id = %s", collection.spec.name, modelID),
  			}
  		}
  		return nil
  	}
  }
docs: // newModelExistsHandler returns a reply handler which will return a
  // ModelNotFound error if the value of reply is false. It is expected to be
  // used as the reply handler for an EXISTS command.