[Sqlalchemy-tickets] Issue #3060: Implicit merge ignores relationship properties set to None (zzzeek/sqlalchemy)

[ods <iss...@bi...>]

New issue 3060: Implicit merge ignores relationship properties set to None
https://bitbucket.org/zzzeek/sqlalchemy/issue/3060/implicit-merge-ignores-relationship

ods:

New object with the same ID as existing one implicitly merged via cascade rules has old value of relationship property after the commit when the new value is None.

In short:
```
#!python
doc.doc_links = [DocLink(id=link_id, link=None)]
session.commit()
assert doc.doc_links[0].link is None  # fails
```

Full test is attached (test3 is the test case that fails).