[5.x] Element relations not saved when creating an element with Craft::createObject()

[nfourtythree]

Description

Noticed a quirk/bug when creating an element using Craft::createObject() and setting data on a element relation custom field.

The test code for creating an entry is as follows:

$randomEntry = Entry::find()->orderBy('RAND()')->one();
$section = Craft::$app->getEntries()->getSectionByHandle('test');

$entry = Craft::createObject([
    'class' => Entry::class,
    'sectionId' => $section->id,
    'title' => 'My Test Entry',
    'enabled' => true,
    'myRelatedEntry' => [$randomEntry->id],
]);

Craft::$app->getElements()->saveElement($entry);

With this code the element saves and if you view the element in the CP or look at its content in the DB everything looks correct. The related element shows up in the field etc.

However, no rows are added to the relations DB table. This means that if you were to do an element query e.g. Entry::find()->relatedTo([123])->one() the entry would not be returned.

Tracked this issue down to two parts.

Because the element is being created with createObject() and mass assignment the $_initialized property on the element is still false when it hits the following bit of code, so the field is not marked as dirty.

cms/src/base/Element.php

Lines 5056 to 5058 in f48af60

	if ($this->_initialized) {
	$this->_dirtyFields[$fieldHandle] = true;
	}

This however doesn't stop the content from saving, but in the case of a relationship field it prevents the relation from saving in the updateRelations() method when figuring out if the field should be included.

cms/src/base/Element.php

Lines 6199 to 6205 in f48af60

	if (
	($this->duplicateOf || $this->isFieldDirty($field->handle) || $field->forceUpdateRelations($this)) &&
	(!$this->propagating || $localizeRelations)
	) {
	$include = true;
	break;
	}

To get the ball rolling on a solution for this I took the idea that if this element is a brand new element we can pass $isNew from the afterSave() to updateRelations(). As, in theory, if it is new we probably want to be trying to save all the data we can.

Related issues

craftcms/commerce#3931