public function keys($values)
    {
        $keys = explode(',', $values);
        $model = $this->builder->getModel();
        $validator = validator([ 'values' => $keys ], [ 'values.*' => 'exists:'.$model->getTable().','.$model->getKeyName() ]);
        return $this->builder->when(!$validator->fails(), function (Builder $query) use($keys) {
            $query->whereKey($keys);
        });
    }