Prefer current Site when checking M2M in "shortcut"/"view_on_site" redirect

[Mike Tigas]

Given an object with a many-to-many relationship to django.contrib.sites.models.Site, the django.contrib.contenttypes.views.shortcut view simply picks the first domain returned, basically obj.<field>[0].domain. (​https://github.com/django/django/blob/4a103086/django/contrib/contenttypes/views.py#L45 -- it even notes that the ordering is arbitrary.)

This can be confusing when operating several (shared database) Django sites where sometimes content is cross-posted between said sites.

Example that led to my noticing this issue: A story being edited on www.spokesman.com but posted to sites [www.spokesman.com, www.downtoearthnw.com] redirects you to story's representation on the latter website when clicking "view on site" in the admin (or using anything else that uses that underlying shortcut view). (Ostensibly because "www.downtoearthnw.com" sorts before "www.spokesman.com" and is therefore picked by the simple code I linked above.) Although this is a theoretically acceptable thing to happen, it's confusing to (even technical) users because they're on one of the acceptable sites for the content object and do not expect to be redirected to another site.

I propose that the view should prefer the current site in the event that both of the following are true:

a) the content object "Site" field is an M2M
b) the current Site object is one of the values set in that M2M

This shouldn't change behavior in any other circumstance (current site not in that M2M, site field is not M2M, etc), but it should alleviate some confusion to this specific usecase.

Have code and will post in a GitHub fork shortly.

[Aymeric Augustin]

This makes sense and is backwards compatible.

[Mike Tigas]

Fork: ​https://github.com/mtigas/django/tree/patches/ticket-18620

Pull request: ​https://github.com/django/django/pull/203

Test isn't the best but does fail without the views.py patch.

[Mike Tigas]

Also I'm on the fence as to whether or not it should optimize and not query for current_site until absolutely necessary (i.e. use lazy or use some if-blocks). Basically to avoid one unnecessary query in the situation if we have a site FK.

[Paulo]

PR: ​https://github.com/django/django/pull/9120

[Tim Graham <timograham@…>]

In d14850e:

Fixed #18620 -- Made ContentTypes shortcut view prefer current site if available.

Thanks Mike Tigas (mtigas) for the initial patch.

[Tim Graham <timograham@…>]

In fa679db1:

Refs #18620 -- Refactored ContentTypes view tests to group related field test cases.