CustomFieldsController::initializeCollection

public function initializeCollection(EntityCollection $collection) : EntityCollection
{
	$filter = $this->buildFilter($collection);
	if ($filter)
	{
		$dbRes = CustomFieldsTable::getList([
			'select' => ['FIELD', 'ENTITY_ID'],
			'filter' => $filter
		]);

		while ($data = $dbRes->fetch())
		{
			$entity = $collection->getItemById($data['ENTITY_ID']);
			$entity->markFieldCustom($data['FIELD']);
		}
	}

	return $collection;
}