protected static function getModelsForRelation($model, $relationKey)
    {
        if (!method_exists($model, $relationKey)) {
            throw new Exception(
                    'Relation "' . $relationKey . '" does not exist in model',
                    static::ERROR_SCOPE | static::ERROR_UNKNOWN_ID,
                    BaseResponse::HTTP_INTERNAL_SERVER_ERROR
                );
        }
        $relationModels = $model->{$relationKey};
        if (is_null($relationModels)) {
            return null;
        }

        if (! $relationModels instanceof Collection) {
            return [ $relationModels ];
        }

        return $relationModels;
    }