> Now I want to get all objects, which have set tag "food" *and* *have
> not* set tag "cake".
>
> This get all objects with tag="food" set (obj1..obj5):
>
> Obj.objects.filter(tags__in=[Tags.objects.filter(name='food')])
>
> This get all objects, which haven't set tag "cake" (obj1..obj3, obj6..obj7):
>
> Obj.objects.exclude(tags__in=[Tags.objects.filter(name='cake')])
>
> When I try to combine DB queries together, I don't get what I expect
> (obj1..obj3, obj6):
Django's ORM doesn't handle this sort of thing very gracefully,
so it requires dropping down to a little SQL with .extra() calls.
Something like
Obj.objects.extra(where=["""
app_model.id IN (
SELECT model_id
FROM app_tags
WHERE name = ?
)
""", ['food']).extra(where["""
app_model.id NOT IN (
SELECT model_id
FROM app_tags
WHERE name = ?
)
""", ['cake'])
it's ugly and hackish, but when things scale up, this is the only
way to get it to work (I've got several hundred thousand records
in my app that does logic like this, so I need all the
speed-boost I can get)
if your tagsets return small collections, you could use
sets...something like
food = set(t.obj_id for t in Tags.objects.filter(name='food'))
cake = set(t.obj_id for t in Tags.objects.filter(name='cake'))
Obj.objects.filter(id__in=food - cake)
(that last line might need to be "tuple(food-cake)")
but as noted, this doesn't scale well once "food" or "cake" tags
thousands of things (or maybe even hundreds of things).
-tim
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