Issue #2986408 by e0ipso, Wim Leers: Use aliasing capabilities provided by JSON API 2.x

src/ResourceType/ConfigurableResourceTypeRepository.php

@@ -253,15 +253,21 @@ class ConfigurableResourceTypeRepository extends ResourceTypeRepository {
     if ($resource_config instanceof NullJsonapiResourceConfig) {
       return [];
     }
-    $entity_type = $resource_config->getEntityType();
-    $bundle = $resource_config->bundle();
+    list($entity_type_id, $bundle) = explode('--', $resource_config->getOriginalId());
+    $entity_type = $this->entityTypeManager->getDefinition($entity_type_id);
     $field_names = $this->getAllFieldNames($entity_type, $bundle);
     // Use the base class to fetch the non-configurable field mappings.
-    $mappings = static::getFieldMapping($field_names, $entity_type, $bundle);
-    // Make sure to respect the overrides coming from JSON:API.
-    array_merge($resource_config->getFieldMapping(), $mappings);
-
-    return $mappings;
+    $mappings = $resource_config->getFieldMapping();
+    // Ignore all the fields that don't have aliases.
+    $mappings = array_filter($mappings, function ($field_info) {
+      return $field_info !== TRUE;
+    });
+    // Make sure to respect the overrides coming from JSON:API if there is no
+    // input in JSON:API Extras.
+    return array_merge(
+      static::getFieldMapping($field_names, $entity_type, $bundle),
+      $mappings
+    );
   }
 
 }