[airstrike]

> ...in very unsuspecting ways

> Trying to access the foreign key of an object by doing `book.user.id` does an additional query for the user table to get the ID. It's less known that the id is immediately available by just doing `book.user_id` instead.

But that's not really unsuspecting. `book.user` is asking for the user table, `book.user_id` is not. Those two things are not identical even though they return the same value.

[cztomsik]

IIRC spark can optimize it away, and in OO there shouldn't be user_id

[airstrike]

> in OO there shouldn't be user_id

I don't think that's right. There is a user_id column in the book table, so why shouldn't there be book.user_id?

[cztomsik]

depends if you're doing active record (which I am not considering as typical ORM) or data mapper like hibernate where your entity is POJO with id and annotations but typically no foreign keys (just references to other model classes)

hibernate is then "the magic "environment where it just works"

I realize OO is out of fashion now but it's still true and it still works and I've been in a lots of projects where ORM was useful