I want to parse an variable into relation, there is an way to use a callback? The variable is the current admin user id.

public $belongsToMany = [
        'users' => [
            'Backend\Models\User',
            'table'    => 'core_cms_user_classes',
            'key'      => 'user_id',
            'otherKey' => 'class_id',
          'conditions' => 'parent_id= $VARIABLE'
        ]
    ];