Parse link fields to get referenced entities

Extracted from a project. At the end of updateRelationships methon in JsonapiHelper.php:

+    foreach ($data['attributes'] as $field_name => $field_data) {
+      if (is_array($field_data) && !array_key_exists(0,$field_data) && isset($field_data['links']['related'])) {
+        $field_values = [];
+        if (isset($field_data['data']['id'])) {
+          $isUnlink = \Drupal::database()->select('entity_share_unlink', 'esu')
+            ->fields('esu', ['entity_id'])
+            ->condition('esu.uuid',$field_data['data']['id']);
+
+          $data = $isUnlink->execute();
+          $unlinkResults = $data->fetchAllAssoc('entity_id');
+          $unlink_nids = array_keys($unlinkResults);
+        }
+        else {
+          $unlink_nids= [];
+        }
+        if(!empty($unlink_nids)) {
+          $field_values = $field_data;
+          $type = explode('--', $field_values['data']['type']);
+          unset($field_values['data']);
+          unset($field_values['links']);
+          $nid = array_shift($unlink_nids);
+          $field_values['uri'] = 'entity:' . $type[0] . '/' . $nid;
+        }
+        else {
+          if (isset($field_data['links']) && isset($field_data['links']['related'])) {
+            $referenced_entities_response = $this->getHttpClient()->get($field_data['links']['related'])
+              ->getBody()
+              ->getContents();
+            $referenced_entities_json = Json::decode($referenced_entities_response);
+
+            if (!isset($referenced_entities_json['errors'])) {
+              $referenced_entities_ids = $this->importEntityListData($referenced_entities_json['data']);
+
+              $field_values = $field_data;
+              $type = explode('--',$field_values['data']['type']);
+              unset($field_values['data']);
+              unset($field_values['links']);
+
+              $field_values['uri'] = 'entity:' .$type[0] . '/' .$referenced_entities_ids[0];
+            }
+          }
+        }
+        $entity->set($field_name, $field_values);
+      }
+      elseif (is_array($field_data)) {
+        $field_datas = $field_data;
+        $field_values = $entity->get($field_name)->getValue();
+        foreach ($field_datas as $index => $field_data) {
+          if (isset($field_data['data']['id'])) {
+            $isUnlink = \Drupal::database()->select('entity_share_unlink', 'esu')
+              ->fields('esu', ['entity_id'])
+              ->condition('esu.uuid',$field_data['data']['id']);
+            $data = $isUnlink->execute();
+            $unlinkResults = $data->fetchAllAssoc('entity_id');
+            $unlink_nids = array_keys($unlinkResults);
+          }
+          else {
+            $unlink_nids= [];
+          }
+          if(!empty($unlink_nids)) {
+            $type = explode('--', $field_data['data']['type']);
+            $nid = array_shift($unlink_nids);
+            $field_values[$index]['uri'] = 'entity:' . $type[0] . '/' . $nid;
+            $field_values[$index]['title'] = $field_data['title'];
+            $field_values[$index]['options'] = $field_data['options'];
+          }
+          else {
+            if (isset($field_data['links']) && isset($field_data['links']['related'])) {
+              $referenced_entities_response = $this->getHttpClient()->get($field_data['links']['related'])
+                ->getBody()
+                ->getContents();
+              $referenced_entities_json = Json::decode($referenced_entities_response);
+
+              if (!isset($referenced_entities_json['errors'])) {
+                $referenced_entities_ids = $this->importEntityListData($referenced_entities_json['data']);
+                $type = explode('--',$field_data['data']['type']);
+                $field_values[$index]['uri'] = 'entity:' .$type[0] . '/' .$referenced_entities_ids[0];
+                $field_values[$index]['title'] = $field_data['title'];
+                $field_values[$index]['options'] = $field_data['options'];
+              }
+            }
+          }
+        }
+        $entity->set($field_name, $field_values);
+      }
+    }

Hello @id.aleks,

Did you have time to work on this issue during the contribution weekend?

To do this feature:

1. Implements an import plugin
2. If think it can be implemented at the prepare_importable_entity_data stage
3. It will parse a link enhanced using the uuid_link field enhancer from JSON:API Extras.
4. From the entity type, bundle and uuid, from the enhanced data, the JSON:API URL of the referenced entity can be requested and imported
5. Maybe also inherit from the entity reference plugin to set a max recursion depth

Hello,

@dgroene Very good MR! Thanks!

Before fixing the review point I will write an automated tests to ensure it is working.

Done, now we should see tests only failing. And with new plugins it will be ok. I have also updated documentation.

And merged!

Thanks!