On Fri, 2006-07-07 at 12:16 +0200, Kilian CAVALOTTI wrote:
> Hi all,
>
> I have a question about reverse queries in ForeignKey relationships. Taking
> the example given in
> <http://www.djangoproject.com/documentation/models/many_to_one/>, I know that
> I can filter the Reporter objects to get those who have associated Articles,
> based on Article properties:
>
> Reporter.objects.filter(article__headline__startswith='This')
>
> I guess I can get all Reporters who have associated Articles by doing:
>
> Reporter.objects.filter(article__isnull = False)
>
>
> But, is there a way to get Reporters who have *no* associated Article? I tried
>
> Reporter.objects.filter(article__isnull = True)
>
> but it returns an empty list, since the article property does not exist for
> those Reporters, and thus, it can't even be null.
>
> The aim is to use this in a limit_choices_to option, to only allow selection
> of 'free' Reporters in the admin interface (ie. Reporters who still do not
> have written any article, they lazy guys).
This is a very interesting question. The answer is kind of hinted at in
the documentation (it's the reverse of creating an empty relation set),
but I only realised that after messing around at the interactive prompt
a little to work out the answer.
Reporter.objects.filter(article = None)
will give you the list you want. I'll add something to the documentation
to make this more explicit once I work out how to phrase it.
Best wishes,
Malcolm
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