I just looked at the resulting SQL and it's pretty subtle:

```
>>> app.authors.filter(user=user).exists()
SELECT (1) AS `a` FROM `users` INNER JOIN `addons_users` ON ( `users`.`id` = `addons_users`.`user_id` ) WHERE (("en-us"="en-us") AND `addons_users`.`addon_id` = 393025  AND `users`.`user_id` = 27 ) LIMIT 1;
True
>>> app.authors.filter(pk=user.pk).exists()
SELECT (1) AS `a` FROM `users` INNER JOIN `addons_users` ON ( `users`.`id` = `addons_users`.`user_id` ) WHERE (("en-us"="en-us") AND `addons_users`.`addon_id` = 393025  AND `users`.`id` = 27 ) LIMIT 1;
True
```

I'm satisfied this query is good and the more correct of the two now. :)