Improve SingleObjectMixin

[tomc]

Belatedly following on from https://code.djangoproject.com/ticket/21898

I feel that slug_url_kwarg and pk_url_kwarg don't feel very in keeping with the rest of django to me, and some thought could be put into improving this part of the API.

My personal opinion is that I don't think that SingleObjectMixin should require a pk, slug, or these variables to be set if you provide a queryset of your own. I don't see any reason why we shouldn't have a DetailView without any arguments in the URL at all, if we provide a single object in some manner. At the moment to do this, we have to override get_object() and essentially rewrite it, which does not feel very elegant to me.

[Aymeric Augustin]

Accepting per mjtamlyn's comment on #21898.

[Alex Myasoedov]

Created pull request:

​https://github.com/django/django/pull/2863

[Tim Graham]

Needs tests and (probably) documentation.

[Danilo Bargen]

Tests have been provided by msoedov.

​https://github.com/django/django/pull/2863

I'm unsure if there's any documentation that needs to be updated... Couldn't find any at a first quick glance, but I could have missed something.

[Tim Graham]

The documentation to be updated is located ​here. Be sure to annotate the changes with .. versionchanged:: 1.8 and also mention it in the release notes. It would also be helpful to describe this better than "Improve SingleObjectMixin." in the ticket & commit message.

[Marc Tamlyn]

Proposed patch was significantly backwards incompatible (see my comments on the PR). It however should be possible to refactor get_object itself to make a better subclassing experience, possibly by adding an inner get_from_queryset method, or by changing the behaviour when no slug or pk is provided.