code: def collections(self, page_size=None):
          
          iterator = self._client._firestore_api.list_collection_ids(
              self._document_path,
              page_size=page_size,
              metadata=self._client._rpc_metadata,
          )
          iterator.document = self
          iterator.item_to_value = _item_to_collection_ref
          return iterator
docs: List subcollections of the current document.
  
          Args:
              page_size (Optional[int]]): The maximum number of collections
              in each page of results from this request. Non-positive values
              are ignored. Defaults to a sensible value set by the API.
  
          Returns:
              Sequence[~.firestore_v1beta1.collection.CollectionReference]:
                  iterator of subcollections of the current document. If the
                  document does not exist at the time of `snapshot`, the
                  iterator will be empty