List top-level collections of the client's database.

        Returns:
            Sequence[~.firestore_v1beta1.collection.CollectionReference]:
                iterator of subcollections of the current document.

def collections(self):
        """List top-level collections of the client's database.

        Returns:
            Sequence[~.firestore_v1beta1.collection.CollectionReference]:
                iterator of subcollections of the current document.
        """
        iterator = self._firestore_api.list_collection_ids(
            self._database_string, metadata=self._rpc_metadata
        )
        iterator.client = self
        iterator.item_to_value = _item_to_collection_ref
        return iterator