Insert document, it must be new if there is ``_id`` in it.

def insert(self, resource, doc_or_docs, **kwargs):
        """Insert document, it must be new if there is ``_id`` in it."""
        ids = []
        kwargs.update(self._es_args(resource))
        for doc in doc_or_docs:
            self._update_parent_args(resource, kwargs, doc)
            _id = doc.pop('_id', None)
            res = self.elastic(resource).index(body=doc, id=_id, **kwargs)
            doc.setdefault('_id', res.get('_id', _id))
            ids.append(doc.get('_id'))
        self._refresh_resource_index(resource)
        return ids