public function addConstraints()
    {
        if (static::$constraints) {
            $query = $this->getRelationQuery();

            $query->where($this->foreignKey, '=', $this->getParentKey());

            $query->whereNotNull($this->foreignKey);
        }
    }

    public function getParentKey()
    {
        return $this->parent->getAttribute($this->localKey);
    }

    public function getAttribute($key)
    {
        if (! $key) {
            return;
        }

        // If the attribute exists in the attribute array or has a "get" mutator we will
        // get the attribute's value. Otherwise, we will proceed as if the developers
        // are asking for a relationship's value. This covers both types of values.
        if (array_key_exists($key, $this->attributes) ||
            array_key_exists($key, $this->casts) ||
            $this->hasGetMutator($key) ||
            $this->isClassCastable($key)) {
            return $this->getAttributeValue($key);
        }

        // Here we will determine if the model base class itself contains this given key
        // since we don't want to treat any of those methods as relationships because
        // they are all intended as helper methods and none of these are relations.
        if (method_exists(self::class, $key)) {
            return;
        }

        return $this->getRelationValue($key);
    }