docs: def merge_parts(self, version_id=None, **kwargs):
          """Merge parts into object version."""
          self.file.update_checksum(**kwargs)
          with db.session.begin_nested():
              obj = ObjectVersion.create(
                  self.bucket,
                  self.key,
                  _file_id=self.file_id,
                  version_id=version_id
              )
              self.delete()
          return obj