public function roles()
    {
        $roleModel = config('defender.role_model', 'Artesaos\Defender\Role');
        $roleUserTable = config('defender.role_user_table', 'role_user');
        $roleKey = config('defender.role_key', 'role_id');

        return $this->belongsToMany($roleModel, $roleUserTable, 'user_id', $roleKey);
    }