Plugin.php

<?php
if (GroupSettings::get('extras_module') == 'hotel') {
    Extra::extend(function ($model) {
        $model->belongsTo['hotel'] = 'BusyRoomsCMS\Group\Models\Hotel';
        $model->rules['hotel'] = 'required';
        $model->attributeNames['hotel'] = 'busyroomscms.group::lang.hotels.labels.hotel';
    });

    Extras::extendListColumns(function ($list, $model) {
        if (!$list->model instanceof Extra) {
            return;
        }

        foreach ($list->getColumns() as $key => $column) {
            $list->removeColumn($key);
        }

        $hotelColumn = [
            'hotel' => [
                'label' => 'busyroomscms.group::lang.hotels.labels.hotel',
                'relation' => 'hotel',
                'select' => 'hotel_name',
            ],
        ];

        $list->addColumns($hotelColumn + $list->columns);
    });

    Extras::extendListFilterScopes(function ($filter) {
        $filter->addScopes([
            'hotel' => [
                'label' => 'busyroomscms.group::lang.hotels.labels.hotel',
                'modelClass' => 'BusyRoomsCMS\Group\Models\Hotel',
                'conditions' => 'hotel_id in (:filtered)',
                'nameFrom' => 'hotel_name',
            ]
        ]);
    });

    Event::listen('backend.form.extendFieldsBefore', function ($widget) {
        if (!$widget->getController() instanceof Extras) {
            return;
        }

        if (!$widget->model instanceof Extra) {
            return;
        }

        if ($widget->isNested) {
            return;
        }

        $hotelField = [
            'hotel' => [
                'label' => 'busyroomscms.group::lang.hotels.labels.hotel',
                'type' => 'relation',
                'nameFrom' => 'hotel_name',
                'placeholder' => 'busyroomscms.group::lang.hints.not_selected',
                'span' => 'right',
                'tab' => 'busyroomscms.content::lang.extras.tabs.basic',
            ],
        ];

        // Add "hotel" field to the second position
        $oldFields = array_get($widget->tabs, 'fields', []);
        $newFields = array_slice($oldFields, 0, 1) + $hotelField + array_slice($oldFields, 1);

        $widget->tabs['fields'] = $newFields;
    });
}