如何在laravel请求文件中提到postgres模式

I'm using Laravel 5.5 and I want to define a unique validation for a table column column_x located inside a Postgres schema schema_x, I've tried :

public function rules()
{
    return [
        'column_x' => 'required|unique:schema_x.table_x'
    ];
}

But the following error keeps raising :

SQLSTATE[42P01]: Undefined table: 7 ERROR: relation "table_x" does not exist

I can't find any information about this case in the official documentation (Validation Section).