code: def list_documents(self, page_size=None):
          
          parent, _ = self._parent_info()
  
          iterator = self._client._firestore_api.list_documents(
              parent,
              self.id,
              page_size=page_size,
              show_missing=True,
              metadata=self._client._rpc_metadata,
          )
          iterator.collection = self
          iterator.item_to_value = _item_to_document_ref
          return iterator
docs: List all subdocuments of the current collection.
  
          Args:
              page_size (Optional[int]]): The maximum number of documents
              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.DocumentReference]:
                  iterator of subdocuments of the current collection. If the
                  collection does not exist at the time of `snapshot`, the
                  iterator will be empty