- Some overdue modification made to allow more flexibility

src/Repository/RelationBuilder.php

@@ -187,7 +187,7 @@ class RelationBuilder
                 return $relation->bridgeField ?? false ? $relation->bridge::entityCollection() : $relationRelation->entity::entityCollection();
         }
 
-        return new $class();
+        throw new \InvalidArgumentException("Unknown or no relation was provided as relation type.");
     }
 
     protected function fetchFromDataset($name, ? array $data = null) : object|bool
@@ -227,6 +227,9 @@ class RelationBuilder
                 elseif ( substr($key, 0, $len ) === sprintf(static::JOIN_FIELD_SEPARATOR, strtolower($name)) ) {
                     $vars[substr($key, $len)] = $value;
                 }
+                else {
+                    # load here for API objects !
+                }
             }
 
             if ($vars) {