Convert Document resource to document ref.

    Args:
        iterator (google.api_core.page_iterator.GRPCIterator):
            iterator response
        item (dict): document resource

def _item_to_document_ref(iterator, item):
    """Convert Document resource to document ref.

    Args:
        iterator (google.api_core.page_iterator.GRPCIterator):
            iterator response
        item (dict): document resource
    """
    document_id = item.name.split(_helpers.DOCUMENT_PATH_DELIMITER)[-1]
    return iterator.collection.document(document_id)