q.py

    select_c = DBSession.query(VPNSession,
                               VPNSession.connect_date.label('ev_date'),
                               sae.literal_column("'c'").label('ev_type')) \
        .order_by(VPNSession.connect_date.desc()) \
        .limit(10)

    select_d = DBSession.query(VPNSession,
                               VPNSession.disconnect_date.label('ev_date'),
                               sae.literal_column("'d'").label('ev_type')) \
        .filter(VPNSession.disconnect_date != None) \
        .order_by(VPNSession.disconnect_date.desc()) \
        .limit(10)

    results = select_c.union(select_d).order_by('ev_date desc').all()