public function addRelation(Relation $relation)
    {
        switch (get_class($relation)) {
            case BelongsTo::class:
                $this->relations['belongsTo'][] = $relation;
                break;

            case BelongsToMany::class:
                $this->relations['belongsToMany'][] = $relation;
                break;

            case HasOne::class:
                $this->relations['hasOne'][] = $relation;
                break;

            case HasMany::class:
                $this->relations['hasMany'][] = $relation;
                break;

            case HasManyThrough::class:
                $this->relations['hasManyThrough'][] = $relation;
                break;

            default:
                throw new \Exception("Unknown relation type " . get_class($relation));
        }
    }