SELECT *
FROM riddle
EXCEPT
SELECT riddle.*
FROM riddle
JOIN result ON result.riddle_id = riddle.id
WHERE result.user_id = 1
riddle = Riddle.query.filter(Riddle.theme == theme_id) \
.except_(Riddle.query.join(Result, Result.riddle_id == Riddle.id) \
.filter(Result.user_id == user.id)).all()