protected function setRelationship($key, $value)
    {
        if (true === $this->isHasOne($key)) {
            return $this->setHasOne($key, $value);
        }
        if (true === $this->isHasMany($key)) {
            throw new \RuntimeException('You cannot set a hasMany relationship directly. Please access using push(), clear(), and/or remove()');
        }
        return $this;
    }