Holy crap, ik heb dit zelf bedacht. Moest alleen de join in de methodedefinitie verwijderen:
def self.from_users_followed_by(user)
followed_user_ids = user.followed_user_ids
where("user_id IN (?) OR user_id = ?", followed_user_ids, user)
end
user.followed_user_ids.join(', ')
produceert dit:"1, 2, 3"
terwijl
user.followed_user_ids
produceert dit:1, 2, 3
dat is wat ik wilde.