The issue is that with `addObject`, we can pass the `Document` (needed to have the right ID) and with `addObjects` we can't. This is an **Elastica** issue.

The only workaround I see is to use the serializer ourselves in the Persister:

```php
    public function insertMany(array $objects)
    {
        $docs = array();
        foreach ($objects as $object) {
            $data = call_user_func($this->_serializer, $object);
            $doc = $this->transformToElasticaDocument($object);
            $doc->setData($data);
            $docs[] = $doc;
        }

        return $this->type->addDocuments($docs);
    }
```

That implies to have the Serializer injected in the `ObjectSerializerPersister` object.

This PR is über important and it's been 5 months now, I would be glad to help with it.