doctrine/doctrine2 — comment on commit 68663fac4b5619643b8a140d6bbe7031e4fa2ad5
File:  line 0
By beberlei:

@IBryukov that does not explain it any better :-) The mapping you show has "owner" the field of the primary key. So it is both nullable and not nullable? owner can never be null in your setup, a primary key is never null.