Fix not supported FOR UPDATE query, fixes #714

[egegunes]

This fixes #714

[coveralls]

Pull Request Test Coverage Report for Build 1101

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.565%

---

Totals
Change from base Build 1095:	0.0%
Covered Lines:	1218
Relevant Lines:	1288

---

💛 - Coveralls

[auvipy]

And it would be nice to add test for the proposed change/

[auvipy]

why select_related is removed?

[egegunes]

update_or_create uses select_for_update in the background if model exists. It's not supported to use select_for_update with nullable relations as mentioned in Django docs.

So, I could change the line to this:

access_token, _created = AccessToken.objects.select_related().update_or_create(...)

But calling select_related with no arguments only follows non nullable relations, nullable relations must be specified (see docs). Since user and application are nullable relations, there is no point to call select_related with no arguments, and we can't specify this fields because of update_or_create.