Nova: Prevent duplicate relation attachment

NovaNoDuplicateRelations.php

<?php

namespace App\Traits\Nova;

use Illuminate\Database\Eloquent\Builder;
use Laravel\Nova\Http\Controllers\AttachableController;
use Laravel\Nova\Http\Requests\NovaRequest;

trait NoDuplicateRelations
{
    /**
     * Build a "relatable" query for the given resource.
     *
     * This query determines which instances of the model may be attached to other resources.
     *
     * @param NovaRequest $request
     * @param Builder $query
     * @return Builder
     */
    public static function relatableQuery(NovaRequest $request, $query): Builder
    {
        if ($request->route()->getController() instanceof AttachableController) {
            $key = $query->getModel()->getQualifiedKeyName();

            return $query->where(function (Builder $builder) use ($key, $query, $request) {
                $builder->whereNotIn($key, function ($subQuery) use ($key, $query, $request) {
                    $subQuery->fromSub(
                        $request->findModelOrFail()->{$request->field}()->select($key)->getQuery(),
                        'tmp'
                    );
                })->when($request->get('current'), function (Builder $builder, $current) use ($key) {
                    $builder->orWhere($key, $current);
                });
            });
        }

        return parent::relatableQuery($request, $query);
    }
}