':param dag_id: DAG ID
:type dag_id: unicode
:param execution_date: execution date
:type execution_date: datetime
:return: DagRun corresponding to the given dag_id and execution date
if one exists. None otherwise.
:rtype: DagRun'

@staticmethod
def get_run(session, dag_id, execution_date):

    qry = session.query(DagRun).filter((DagRun.dag_id == dag_id), (DagRun.external_trigger == False), (DagRun.execution_date == execution_date))
    return qry.first()
