@overload
    def _get_key(self, object_type: str, object_: Union[Revision, Release, Directory, Snapshot]) -> bytes:
        ...

    @overload
    def _get_key(self, object_type: str, object_: Content) -> bytes:
        ...

    @overload
    def _get_key(self, object_type: str, object_: SkippedContent) -> Dict[str, bytes]:
        ...

    @overload
    def _get_key(self, object_type: str, object_: Origin) -> Dict[str, bytes]:
        ...

    @overload
    def _get_key(self, object_type: str, object_: OriginVisit) -> Dict[str, str]:
        ...

    def _get_key(self, object_type: str, object_: BaseModel) -> Union[bytes, Dict]:
        if object_type in ("revision", "release", "directory", "snapshot"):
            return object_.id
        elif object_type == "content":
            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":
            return {"url": object_.url}
        elif object_type == "origin_visit":
            return {
                "origin": object_.origin,
                "date": str(object_.date),
            }
        else:
            raise ValueError("Unknown object type: %s." % object_type)