def _get_key(
            self,
            object_type: str,
            object_: BaseModel) -> Union[bytes, Dict]:
        if object_type in ('revision', 'release', 'directory', 'snapshot'):
            object_ = cast(
                Union[Revision, Release, Directory, Snapshot],
                object_)
            return object_.id
        elif object_type == 'content':
            object_ = cast(Content, object_)
            return object_.sha1  # TODO: use a dict of hashes
        elif object_type == 'skipped_content':
            return {
                hash: getattr(object_, hash)
                for hash in DEFAULT_ALGORITHMS
            }
        elif object_type == 'origin':
            object_ = cast(Origin, object_)
            return {'url': object_.url}
        elif object_type == 'origin_visit':
            object_ = cast(OriginVisit, object_)
            return {
                'origin': object_.origin,
                'date': str(object_.date),
            }
        else:
            raise ValueError('Unknown object type: %s.' % object_type)