EntityReferenceFieldItemNormalizerTrait.php

<?php

namespace Drupal\serialization\Normalizer;

use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;

/**
 * Converts empty reference values for entity reference items.
 */
trait EntityReferenceFieldItemNormalizerTrait {
    protected function normalizeRootReferenceValue(&$values, EntityReferenceItem $field_item) {
        // @todo Generalize for all tree-structured entity types.
        if ($this->fieldItemReferencesTaxonomyTerm($field_item) && empty($values['target_id'])) {
            $values['target_id'] = NULL;
        }
    }
    
    /**
     * Determines if a field item references a taxonomy term.
     *
     * @param \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $field_item
     *
     * @return bool
     */
    protected function fieldItemReferencesTaxonomyTerm(EntityReferenceItem $field_item) {
        return $field_item->getFieldDefinition()
            ->getSetting('target_type') === 'taxonomy_term';
    }

}