Re: .filter() and .exclude() don't add up

On Sunday, July 28, 2013 10:44:49 AM UTC+3, Daniele Procida wrote:

On Sat, Jul 27, 2013, Steve McConville <mcconvil...@gmail.com> wrote:

>Perhaps I'm not sure exactly what you mean by "more general", but I
>was recommending something like
>
>red_things = queryset.filter(Q(color="red"))
>non_red_things = queryset.filter(~Q(color="red") | Q(color__isnull=True)
>
>This will produce SQL like
>
>SELECT * FROM queryset WHERE color IS 'red';
>SELECT * FROM queryset WHERE color IS NOT 'red' OR color IS NULL;
>
>The set non_red_things will be the complement of red_things.

I understood that part. But by "more general" I mean one that will work for any case, without having to know where the Nulls might be.

So given queryset A, and its subset queryset B, we can place B against A and obtain its complement.

Or to put it another way: give me all the items in A that are not in B.

You can do this with a subquery in Django. non_red_things = queryset.exclude(pk__in=red_things). If this performs well is a different thing.

I think that in SQL one can use WHERE (original_condition) is not true; which will match both unknown (null comparison's result) and false in the original condition.

 - Anssi

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