Make setting field values via Entry::EVENT_BEFORE_SAVE work when $firstSave is true

[i-just]

Description

Setting field values via Entry::EVENT_BEFORE_SAVE doesn’t work when $firstSave is true.

This happens (in v5 only) because in _saveElementInternal(), we no longer always save the element’s content.

Example code:

Event::on(
    Entry::class,
    Entry::EVENT_BEFORE_SAVE,
    function (ModelEvent $event) {
        $entry = $event->sender;

        if ($entry->sectionId == 14 && $entry->firstSave) {
            $entry->setFieldValues([
                'number' => 6,
            ]);
        }
    }
);

The order of operations is: ElementsController->actionApplyDraft() is called, then Elements->_saveElementInternal(). At that point, $element->firstSave is false, so when EVENT_BEFORE_SAVE is called, the condition is not met, and the field values are not set. The $saveContent is true at this point, so content is saved, but since field values weren’t set, this doesn’t result in the desired change.

Then Drafts->applyDraft() gets called, then Drafts->removeDraftData(), and that’s what sets $element->firstSave to true. However, when it later calls Elements->_saveElementInternal(), it calls it with $saveContent set to false, so EVENT_BEFORE_SAVE is called, the condition is met, and the field values are set, but the content is not actually saved.

Related issues

#15369

[brandonkelly]

The only point of that saveElement() call from removeDraftData() is to get draftId removed from the element. We’re explicitly saying we don’t want content to be saved, so I think saveElement() should respect that, even if a plugin changes a field value in the process.

Maybe we should just start communicating the intention by adding a saveContent property on the element?