#16025: distinct does not apply to aggregated querysets
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Reporter: micolous | Owner: nobody
Type: Bug | Status: new
Milestone: | Component: Database layer (models, ORM)
Version: 1.3 | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Easy pickings: 0
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When applying .distinct() to a QuerySet, performing an aggregation using
.aggregate() on that will only ensure that the aggregation result is
distinct. As a result, this prevents you from doing a query where you
want the input to the aggregation to have distinct rows.
Take the example models:
{{{
class Category(Model):
label = CharField(max_length=128)
class Food(Model):
name = CharField(max_length=128)
categories = ManyToManyField(Category)
kilojoules_per_kg = PositiveIntegerField()
}}}
If you then did a query on which you selected a couple of Categories as
foreign keys (eg: "Pizza Toppings" and "Fruits"), you'd end up with a list
of Foods that would contain duplicates where a food item fits in multiple
categories (eg: "Apple", "Orange", "Pineapple", "Pineapple", "Bacon" and
"Cheese" ...).
If you then wanted to then find the average number of kilojoules per
kilogram for items in those groups and eliminate duplicates (Pineapple),
the documentation would lead you to do this:
{{{
qs.distinct().aggregate(Average('kilojoules_per_kg'))
}}}
However instead of counting Pineapple once, Pineapple would be counted
twice, as the distinct() call would only apply to the result of the
aggregation, not to the input of it.
{{{
>>> qs.distinct().aggregate(Average('kilojoules_per_kg')) ==
qs.aggregate(Average('kilojoules_per_kg'))
True
}}}
Instead, .distinct() before .aggregate() should instead cause a subquery
to be executed to filter the results before passing it to the aggregation
functions. There doesn't seem a way to presently execute a query like
this using the Django ORM.
.distinct() after an aggregation should cause the results of the
aggregation to be filtered so only distinct records are returned (which is
the current behavior).
I've managed to confirm this behavior in Django 1.2.3 and 1.3.
--
Ticket URL: <http://code.djangoproject.com/ticket/16025>
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