find common elements in in a many-to-many helper table

many-to-many-common-finder.py

	def find_common(*user_ids):
		queries = []
		for user_id in user_ids:
			query = db.session.query(participants.c.thread_id) \
				.filter(participants.c.user_id==user_id)
			queries.append(query)
		thread_ids = [r[0] for r in db.session.query(participants.c.thread_id) \
			.intersect(*queries).all()]
		threads = Thread.query.filter(Thread.id.in_(thread_ids)).all()
		return threads