'Returns the dag run for a given execution date if it exists, otherwise
none.
:param execution_date: The execution date of the DagRun to find.
:param session:
:return: The DagRun if found, otherwise None.'

@provide_session
def get_dagrun(self, execution_date, session=None):

    dagrun = session.query(DagRun).filter((DagRun.dag_id == self.dag_id), (DagRun.execution_date == execution_date)).first()
    return dagrun
