#19415: Clarify how aggregates work with multi-valued relationships and multiple
filter() calls
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Reporter: svniemeijer@… | Owner:
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
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Comment (by benred42):
I did some more digging on this issue as I was working on editing my Doc
patch and I learned a bit more about what's going on. Consider the
following test case:
{{{
class Author(models.Model):
name = models.CharField(max_length=10)
class Book(models.Model):
title = models.TextField()
pages = models.IntegerField(null=True)
rating = models.IntegerField(null=True)
authors = models.ManyToManyField(Author)
a1 = Author.objects.create(name='a1')
a2 = Author.objects.create(name='a2')
a3 = Author.objects.create(name='a3')
a4 = Author.objects.create(name='a4')
b1 = Book.objects.create(title='b1', pages=100, rating=4)
b1.authors.set([a1])
b2 = Book.objects.create(title='b2', pages=50, rating=2)
b2.authors.set([a2])
b3 = Book.objects.create(title='b3', pages=150, rating=3)
b3.authors.set([a4])
b4 = Book.objects.create(title='b4', pages=400, rating=5)
b4.authors.set([a4])
b5 = Book.objects.create(title='b5', pages=100, rating=1)
b5.authors.set([a1,a4])
}}}
Now, let's look at two queries on that test case:
{{{
qs1 = Author.objects.filter(book__pages__lt=200,
book__pages__gt=100).annotate(num_book=Count('book'))
qs2 =
Author.objects.filter(book__pages__lt=200).filter(book__pages__gt=100).annotate(num_book=Count('book'))
}}}
The first query gives us the expected results:
{{{
for i in qs1:
print(i, i.num_book)
>>> a4, 1
print(qs1.query)
>>> SELECT "queries_author"."id", "queries_author"."name",
"queries_author"."num", "queries_author"."extra_id",
COUNT("queries_book_authors"."book_id") AS "num_book" FROM
"queries_author" INNER JOIN "queries_book_authors" ON
("queries_author"."id" = "queries_book_authors"."author_id") INNER JOIN
"queries_book" ON ("queries_book_authors"."book_id" = "queries_book"."id")
WHERE ("queries_book"."pages" < 200 AND "queries_book"."pages" > 100)
GROUP BY "queries_author"."id", "queries_author"."name",
"queries_author"."num", "queries_author"."extra_id" ORDER BY
"queries_author"."name" ASC
}}}
The second query, however, gives some rather strange results (author4 only
has 3 books, not 4!):
{{{
for i in qs2:
print(i, i.num_book)
>>> a4, 4
print(qs2.query)
>>> SELECT "queries_author"."id", "queries_author"."name",
"queries_author"."num", "queries_author"."extra_id",
COUNT("queries_book_authors"."book_id") AS "num_book" FROM
"queries_author" INNER JOIN "queries_book_authors" ON
("queries_author"."id" = "queries_book_authors"."author_id") INNER JOIN
"queries_book" ON ("queries_book_authors"."book_id" = "queries_book"."id")
INNER JOIN "queries_book_authors" T4 ON ("queries_author"."id" =
T4."author_id") INNER JOIN "queries_book" T5 ON (T4."book_id" = T5."id")
WHERE ("queries_book"."pages" < 200 AND T5."pages" > 100) GROUP BY
"queries_author"."id", "queries_author"."name", "queries_author"."num",
"queries_author"."extra_id" ORDER BY "queries_author"."name" ASC
}}}
So, the problem appears to be that `Count` is only looking at results from
`queries_book_authors` and cannot see results from the aliased tables
(`T4` and `T5`), thus it only counts from the first joins and not the
latter joins. This results, rather bizarrely, in the annotated value
being the ''product'' of the number of results meeting the first condition
(`"queries_book"."pages" < 200`) times the number of results meeting the
second condition (`T5."pages" > 100`). This result is neither intuitive
nor helpful, so I would suggest that, going forward, the documentation
should provide an example of the first query and a warning not to annotate
queries with multiple filters as in the second query since the results are
not what would be expected or useful.
NOTE: I have not yet tested this case with just a ForeignKey relationship,
so as of now the above statements can only be applied when filtering
across M2M relationships.
--
Ticket URL: <https://code.djangoproject.com/ticket/19415#comment:16>
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