yii\db\ActiveRelationTrait::getModelKey PHP Method

    private function getModelKey($model, $attributes)
    {
        $key = [];
        foreach ($attributes as $attribute) {
            $key[] = $this->normalizeModelKey($model[$attribute]);
        }
        if (count($key) > 1) {
            return serialize($key);
        }
        $key = reset($key);
        return is_scalar($key) ? $key : serialize($key);
    }