EmbedsMany respect primaryKey on association

src/Jenssegers/Mongodb/Relations/EmbedsMany.php

@@ -236,7 +236,7 @@ class EmbedsMany extends EmbedsOneOrMany
     protected function associateNew($model)
     {
         // Create a new key if needed.
-        if (!$model->getAttribute('_id')) {
+        if ($model->getKeyName() == '_id' && !$model->getAttribute('_id')) {
             $model->setAttribute('_id', new ObjectID);
         }