Field: Selectable Entries Condition -> get all descendants of certain id

Question

See: https://craftcms.com/docs/4.x/entries.html#descendantof

I want to create a field which allows to select all (direct) descendants of a certain entry (id=306), section type = structure.

My solution: {craft.entries().descendantOf(306).all()}

Result: I got all entries ... not only the wanted descendants ... but why?
I've no idea ... it should work - but it does not.

Further testing and tryout: A dump in a twig template works fine:
{% dump(craft.entries().descendantOf(306).all()) %}

Screenshots

Field configuration

Section

wrong result -> too many entries

Answer 1

Parent/child (aka ancestor/descendant) relationships aren't relations, which is what the "Related to" (and the relatedTo query param underpinning it) works with. In other words, it's not possible to create a "Related to" condition that does what you need it to.

Unfortunately, neither Craft 4 or 5 has a "Descendant of" condition type. Sounds like a valid feature request, though!

If you really want to pull this off, the only solution I can think of is to create a custom Yii module, with an event listener for the EVENT_DEFINE_SELECTION_CRITERIA event:

use yii\base\Event;
use craft\fields\BaseRelationField;
use craft\events\ElementCriteriaEvent;

Event::on(
    BaseRelationField::class,
    BaseRelationField::EVENT_DEFINE_SELECTION_CRITERIA,
    function (ElementCriteriaEvent $event) {
        $field = $event->sender;
        if ($field->handle !== 'entryFieldHandle') {
            return;
        }
        $event->criteria['descendantOf'] = 306;
    }
);

Finally, as general advice you should avoid hard coding entry IDs anywhere in your build. Entry IDs will typically vary between environments, so a better approach is to either use environment variables, or an entry query to find the proper entry without depending on a hard coded ID.

Answer 2

I see ... thanks a lot, Mats!

I modified your code a little bit (thanks to ChatGPT), now it works for multiple parent ids (Craft 4):

custom.php

[...]
'customParentIds' => [306, 556],
[...]

module

use yii\base\Event;
use craft\fields\BaseRelationField;
use craft\events\ElementCriteriaEvent;
use craft\elements\Entry;
use Craft;

Event::on(
    BaseRelationField::class,
    BaseRelationField::EVENT_DEFINE_SELECTION_CRITERIA,
    function (ElementCriteriaEvent $event) {
        $field = $event->sender;

        // check field handle
        if ($field->handle !== 'entryFieldHandle') {
            return;
        }

        // get customParentIds
        $parentIds = Craft::$app->config->custom->customParentIds;

        // merge descendant IDs
        $descendantIds = [];
        foreach ($parentIds as $parentId) {
            $descendants = Entry::find()->descendantOf($parentId)->level(2)->ids();
            $descendantIds = array_merge($descendantIds, $descendants);
        }

        // set the ID criteria for the event
        $event->criteria['id'] = $descendantIds;
    }
);