silverstripe-frameworktest/code/linkfield/Extensions/ElementContentExtension.php

<?php

namespace SilverStripe\FrameworkTest\LinkField\Extensions;

use SilverStripe\Core\Extension;
use SilverStripe\LinkField\Models\Link;
use SilverStripe\LinkField\Models\EmailLink;
use SilverStripe\LinkField\Models\PhoneLink;
use SilverStripe\LinkField\Models\SiteTreeLink;

class ElementContentExtension extends Extension
{
    private static bool $inline_editable = true;

    private static array $has_one = [
        'OneLink' => Link::class,
    ];

    private static $has_many = [
        'ManyLinks' => Link::class . '.Owner',
    ];

    private static array $owns = [
        'OneLink',
        'ManyLinks',
    ];

    private static array $cascade_deletes = [
        'OneLink',
        'ManyLinks',
    ];

    private static array $cascade_duplicates = [
        'OneLink',
        'ManyLinks',
    ];

    protected function updateCMSFields($fields)
    {
        $fields->dataFieldByName('OneLink')->setAllowedTypes([
            SiteTreeLink::class,
            EmailLink::class,
            PhoneLink::class
        ]);
    }
}