How to use relationExtendConfig properly?

I want to extend the Orders for Shopaholic plugin, so the Offer list in Order has some more information from the parent product. I am trying to use relationExtendConfig, the Order model has the relationBehavior and config but I am getting Call to undefined method Lovata\OrdersShopaholic\Controllers\Orders::relationExtendConfig() so I am not sure if I need to extend the controller or what? Thank you

Hi @Vosco

This is a controller method override. You may need to extend the List widget globally and filter it down to determine if it happens inside the relation. Here is an example:

Event::listen('backend.list.extendColumns', function ($listWidget) {
    // Only for the User controller
    if (!$listWidget->getController() instanceof \Backend\Controllers\Users) {

    // Only for the User model
    if (!$listWidget->model instanceof \Backend\Models\User) {

    // Add an extra birthday column
        'birthday' => [
            'label' => 'Birthday'

    // Remove a Surname column