Fix issue with multiple entity references

Closes #3438180

src/Normalizer/RdfSyncNormalizer.php

@@ -70,7 +70,7 @@ public function normalize($object, $format = NULL, array $context = []): array {
     }
 
     foreach ($normalized as $fieldName => $items) {
-      foreach ($items as $item) {
+      foreach ($items as $delta => $item) {
         foreach ($item as $columnName => $value) {
           if (!$mapping = ($mappings[$fieldName][$columnName] ?? NULL)) {
             // This column is not mapped.
@@ -87,17 +87,19 @@ public function normalize($object, $format = NULL, array $context = []): array {
               $value = $rdfType;
             }
           }
-          // Resolve references to other entities.
-          elseif (is_a(EntityReferenceItem::class, $itemDefinition->getClass(), TRUE)) {
-            /** @var \Drupal\Core\Entity\EntityInterface $entity */
-            if (!$entity = $object->get($fieldName)->entity) {
+          // Resolve references to other entities. Cover also field types that
+          // are extending entity reference.
+          elseif ($columnName === 'target_id' && is_a($itemDefinition->getClass(), EntityReferenceItem::class, TRUE)) {
+            /** @var \Drupal\Core\Entity\ContentEntityInterface $targetEntity */
+            $targetEntity = $object->get($fieldName)->get($delta)->entity;
+            if (!$targetEntity) {
               $value = NULL;
             }
-            elseif ($this->mapper->isMappedEntity($entity)) {
+            elseif ($this->mapper->isMappedEntity($targetEntity)) {
               // If the destination is also mapped, replace the scalar ID (int,
               // string) with the URI.
-              $targetUriFieldName = $this->mapper->getRdfUriFieldName(entity: $entity);
-              $value = $object->get($fieldName)->entity->get($targetUriFieldName)->value;
+              $targetUriFieldName = $this->mapper->getRdfUriFieldName(entity: $targetEntity);
+              $value = $targetEntity->get($targetUriFieldName)->value;
             }
           }
 

[Claudiu Cristea]

By not using delta, on each item $entity is the 1st referred item in the list (delta === 0). This is the source of the bug.