Discussion on: Why is Rails ActiveRelation.update_all updating a different set of records?

You probably hit a limitation of ActiveRecord :-)

union is not part of ActiveRecord, which might be the reason why update_all is not able to work with it.

There's a lot of magic in it :D

PostgreSQL itself is able to update rows from unions if you project the same exact row types from both queries:

PracticalDeveloper_development> select count(*) from comments;
+---------+
| count   |
|---------|
| 30      |
+---------+
PracticalDeveloper_development> update comments set updated_at = now() where id in (select id from comments where id < 10 union select id from comments where id > 15);
UPDATE 24

In conclusion: I don't know what's going on 🧐