<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_citation_by_hash(self, citation_hash: str) -> Optional[Citation]: """Get a citation object by its hash."""

        return self.session.query(Citation).filter(Citation.sha512 == citation_hash).one_or_none()