code: def collection(self, collection_id):
          
          child_path = self._path + (collection_id,)
          return self._client.collection(*child_path)
docs: Create a sub-collection underneath the current document.
  
          Args:
              collection_id (str): The sub-collection identifier (sometimes
                  referred to as the "kind").
  
          Returns:
              ~.firestore_v1beta1.collection.CollectionReference: The
              child collection.