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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def polymorphic_update_timestamp(session, flush_context, instances):
 """This listener ensures an update statement is emited for "entity" table to update 'updated_at'. With joined-table inheritance if the only modified attributes are subclass's ones, then no update statement will be emitted. """

    for obj in session.dirty:
        if not isinstance(obj, Entity):
            continue
        state = sa.inspect(obj)
        history = state.attrs["updated_at"].history
        if not any((history.added, history.deleted)):
            obj.updated_at = datetime.utcnow()