event = Event.get_one(<event_id>)
# get all abstracts submitted for a single track
Abstract.query.with_parent(event).filter(Abstract.submitted_for_tracks.any(Track.id == <track_id>)).all()
# get all abstracts submitted to multiple tracks
Abstract.query.with_parent(event).filter(Abstract.submitted_for_tracks.any(Track.id.in_([<list of track ids>]))).all()