/**
* {@inheritdoc}
*/
public function getConstraints()
{
$constraints = parent::getConstraints();
// If this data definition is marked as required for the sake of schema
// definitions, we don't enforce it using the NotNull constraint. Instead
// \Drupal\Core\Field\EntityReferenceItem is validated by the
// 'ValidReference' constraint that operates at the field-item level. This
// constraint takes into consideration that the target_id property can
// be derived from the entity property.
unset($constraints['NotNull']);
return $constraints;
}
