ModelAdmin hook for customising the "show on site" button

As Simon Willison ​suggested on django developers:

Further thinking about get_absolute_url. At the moment, the main 
reason it's a standard in Django is that it is used by 
django.contrib.admin to display the "show on site" button inside the 
admin interface. 

This mechanism isn't very robust. Admin sites are often run on 
different subdomains from the rest of the site, which means that 
get_absolute_url on its own isn't enough - instead, we have to go 
through the whole /r/{{ content-type-id }}/{{ object-id }}/ palaver 
which attempts to figure out the site based on ForeignKey(Site) 
properties on the model, or by using the default site from SITE_ID. 

It would make a lot more sense to allow ModelAdmin subclasses to 
define that behaviour themselves, for example: 

class ArticleAdmin(admin.ModelAdmin): 
    ... 
    def show_on_site(self, obj): 
        return 'http://livesite.example.com' + obj.get_url_path() 

We could even support "show_on_site = True" as meaning "include the 
show on site link, using cleverness to figure out the link" whereas 
show_on_site = a_callable means "call this function to figure out the 
URL for that link".

[redduck666]

i wonder, am i on the right track?

[Ivan Giuliani]

Attached patch implements view_on_site as either a boolean flag or a callable. There are two issues I had to deal with:

• I made default for view_on_site False since we're not relying on get_absolute_url anymore, so there's no way to effectively know whether we have a URL where we can go or not;
• I had to add the view_on_site_url parameter to InlineAdminForm since there was no way to access to the InlineModelAdmin object from there. This could break backward compatibility if someone was inheriting from InlineModelForm (that's not documented anywhere though);

Apart from this, patch includes tests and documentation.

[patchhammer]

view_on_site.diff fails to apply cleanly on to trunk

[Unai Zalakain]

PR: ​https://github.com/django/django/pull/1804

[Simon Charette]

Added some comments on the PR.

[Unai Zalakain]

Proposed changes applied!

[Simon Charette]

Finished my review, sorry for the delayed comments.

[Unai Zalakain]

It doesn't matter, PR updated!

[Simon Charette]

Patch looks RFC, please rebase and I'll merge it.

[Unai Zalakain]

Tim Grahams suggestions taken into account and rebased master on it!

[Unai Zalakain]

Once again!

[Tim Graham]

This looks good to me as well.

[Simon Charette <charette.s@…>]

In fd219fa24c7911adab60e1f5e4fd3d7f9d82a969:

Fixed #8261 -- ModelAdmin hook for customising the "show on site" button

ModelAdmin.view_on_site defines wether to show a link to the object on the
admin detail page. If True, cleverness (i.e. Model.get_absolute_url) is
used to get the url. If it's a callable, the callable is called with the object
as the only parameter. If False, not link is displayed.

With the aim of maitaining backwards compatibility, True is the default.