A question on forms; I had asked something similar on StackOverflow
but found the solution didn't really work for me. Let's say I have an
Article class and a Photo class, and I want to be able to attach
Photos to Articles. I create an ArticlePhotoSet class, and make it an
InlineForm in the Article's Admin class. Here's the problem:

Articles can be assigned to a site, and Photos can be assigned to a
site. Each time an Article is saved in the admin, the form needs to
throw an error if any of the Photos pointed to by the ArticlePhotoSet
inlines don't belong to the same site(s) that the Article does. The
problem is that in the ArticlePhotoSet formset, self.instance in the
clean() function refers to what is currently saved for the article,
not what the Article form is 'proposing' the new site be in the form.
So any check placed in the ArticlePhotoSet formset clean() is
referring to what the site value is, not what it will be. Conversely,
the formset doesn't seem to be accessible from the cleaning method of
the Article form itself, so the Article's form can't do the check. Is
there a way to do what I'm trying to accomplish without falling back
onto something more extreme like AJAX checks when the Article's site
value is changed on the form?