Django filter by Count annotated inside Subquery

Question

Here is an abstract and simplified example.

Say I want to get authors and annotate minimal count of books in category if it is greater than three.

Book and Author models and are not connected with ForeignKey fields (remember, abstract and simplified, there is a reason):

Author(models.Model):
    name = models.CharField(max_length=250)

Book(models.Model):
    author_name = models.CharField(max_length=250)
    book_category = models.CharField(max_length=250)

Here is simplest query I can get to reproduce:

(Author.objects
 .annotate(min_valuable_count=Subquery(
    Book.objects
        .filter(author_name=OuterRef('name'))
        .annotate(cnt=Count('book_category'))
        .filter(cnt__gt=3)
        .order_by('cnt')
        .values('cnt')[:1],
    output_field=models.IntegerField()
 ))
)

And I get an error:

psycopg2.ProgrammingError: missing FROM-clause entry for table "U0"
LINE 1: ... "core_author" GROUP BY "core_author"."id", "U0"."id" ...
                                                       ^

Here is SQL:

SELECT "core_author"."id", "core_author"."name", (
    SELECT COUNT(U0."book_category") AS "cnt" 
    FROM "core_book" U0 WHERE U0."id" = ("core_author"."chat_id") 
    GROUP BY U0."id" HAVING COUNT(U0."book_category") > 3 
    ORDER BY "cnt" ASC  LIMIT 1) 
AS "min_valuable_count" 
FROM "core_author" 
GROUP BY "core_author"."id", "U0"."id"

Update #1

I found that deleting .filter(cnt__gt=3) removes last GROUP BY which has no access to U0:

SELECT "core_author"."id", "core_author"."name", (
    SELECT COUNT(U0."book_category") AS "cnt" 
    FROM "core_book" U0 WHERE U0."id" = ("core_author"."chat_id") 
    GROUP BY U0."id"
    ORDER BY "cnt" ASC  LIMIT 1) 
AS "min_valuable_count" 
FROM "core_author"

Is there any way to remove GROUP BY in outer query without removing .filter(cnt__gt=3) in subquery?

Dan Zaikin · Accepted Answer · 2019-01-14 21:10:22Z

3

This is bug and it will be fixed in django version 2.1.6. Here is workaruond for now:

min_valuable_count_qs = Subquery(
    Book.objects
        .filter(author_name=OuterRef('name'))
        .annotate(cnt=Count('book_category'))
        .filter(cnt__gt=3)
        .order_by('cnt')
        .values('cnt')[:1],
    output_field=models.IntegerField()
)

min_valuable_count_qs.contains_aggregate = False

qs = Author.objects.annotate(min_valuable_count=min_valuable_count_qs))

edited Jan 14, 2019 at 21:10

answered Jan 12, 2019 at 14:02

Dan Zaikin

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