feat(resource): Add childTemplate property for default child template

[Ibochkarev]

What does it do?

Adds support for the childTemplate template property. When creating a new child resource under a parent, if the parent's template has a childTemplate property set to a template ID, that template is preselected as the default for the new resource (both in the create form and when saving).

Backend (Resource/Create processor): resolveDefaultTemplate() and getChildTemplateIdFromParent() read the parent template's childTemplate property and use it when no template is explicitly provided.

Manager (resource/create controller): getChildTemplateIdFromParentTemplate() sets the default template in the create form. Takes precedence over automatic_template_assignment when childTemplate is set.

Why is it needed?

Allows template authors to define which template should be used for child resources (e.g. News overview → News detail), reducing manual template selection when creating documents in a hierarchy.

How to test

1. Add a childTemplate property to a template (value = template ID).
2. Create a child resource under a resource that uses that template.
3. Confirm the child template is preselected in the form and saved correctly.

Related issue(s)/PR(s)

Resolves #13753

[biz87]

Code Review

Summary

Adds childTemplate template property support — when creating a child resource, the template defined in the parent template's childTemplate property is preselected. Changes in Resource/Create.php processor and resource/create.class.php controller, +143/-26.

Issues

The getChildTemplateIdFromParent() logic is nearly duplicated across two files — the processor (Create.php:330-383) and the controller (create.class.php:313-355). Differences are minor: return type (?int vs int), parent property name, and existence check. This is a DRY violation — a bug fix or behavior change would need updating in both places. Consider extracting the logic into modTemplate as a method (e.g. getChildTemplateId(modResource $parent)).

getObject is used solely to check template existence (Create.php:374, create.class.php:347). A full object load for an existence check is wasteful — $this->modx->getCount(modTemplate::class, $templateId) > 0 would suffice.

Assessment

Useful feature, well-structured with early returns. But the code duplication is a significant maintainability concern.

Verdict

Request changes — extract shared logic to eliminate duplication.