Can I count all children in Queryset annotate?

[dimagi]

Can I count all children in Queryset annotate?

[millerdev]

I'm guessing a bit at what you're asking, but yes, that should be possible.

cte = With(
    Order.objects
    .values("region_id")
    .annotate(number=Count("id"))
)

orders = (
    cte.join(Order, region=cte.col.region_id)
    .with_cte(cte)
    .annotate(region_count=cte.col.number)
)

[johnvonneumann7]

Thank you. I assume this will work successfully.
I need to get the data for the other columns at the same time.

What if I need to include this number in the parent data to get a listing as shown below?

[
  {
    "id": 1,
    "text": "...",
    "reply_count": 10,
  },
  {
    "id": 2,
    "text": "...",
    "reply_count": 7,
  },
  {
    "id": 3,
    "text": "...",
    "reply_count": 3,
  },
]

[millerdev]

You haven't listed the models (tables) you're working with or their relationships, and I'd prefer not to keep guessing. Can you provide the SQL query you want to produce? That will make it easier to tell if you can create the same query with django-cte.

[johnvonneumann7]

Sorry. The following are those.

To rearrange again, we need to get rows where the parent column is null, and
need to annotate the number of all its descendants
(all descendants of the hierarchy, not just the immediate children)

class CustomQuerySet(CTEQuerySet):
    ...


class Comment(models.Model):
    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    text = models.TextField(max_length=1000)
    parent = models.ForeignKey(
        "self",
        blank=True,
        null=True,
        on_delete=models.CASCADE,
        related_name="replies",
    )

    objects = CustomQuerySet.as_manager()

I don't know the SQL statement that accomplishes what I want to do, but if I want to get the number of immediate children only
Here is what it would look like

Comment.objects.filter(post_id=post).annotate(reply_count=Count("replies"))

SELECT "posts_comment"."id",
       "posts_comment"."text",
       "posts_comment"."parent_id",
       "posts_comment"."post_id",
       COUNT(T3."id") AS "reply_count"
FROM "posts_comment"
LEFT OUTER JOIN "posts_comment" T3 ON ("posts_comment"."id" = T3."parent_id")
WHERE ("posts_comment"."post_id" = 465a22f4-12de-4a4e-a294-88f4abfdec5c
       AND "posts_comment"."parent_id" IS NULL)
GROUP BY "posts_comment"."id"

[millerdev]

You can use a Recursive Common Table Expression to solve that problem. This sounds a bit like a homework problem, so I am stopping short of giving the answer.

[johnvonneumann7]

Of course, I know how to use the basic recursive common table expression, and I can get all descendants of the parent table, but I don't know how to annotate the number of all descendants (how to use a recursive common table expression in annotate?)

I don't know how to code the "reply_count=???"

Comment.objects.filter(parent__isnull=True).annotate(reply_count=???)

I know that the code to retrieve all descendants would look like this

def make_cte(cte):
    return (
        Comment.objects.filter(parent=parent)
        .union(cte.join(Comment, parent_id=cte.col.id))
    )

cte = With.recursive(make_cte)

return cte.queryset().with_cte(cte)

[johnvonneumann7]

I tried to annotate reply_count in this way, but reply_count is null for some reason.

def make_cte(cte):
    return Comment.objects.filter(parent=OuterRef("pk")).union(
        cte.join(Comment, parent_id=cte.col.id)
    )

cte = With.recursive(make_cte)

subquery = Subquery(
    cte.queryset()
    .with_cte(cte)
    .annotate(count=Count("pk"))
    .values("count"),
    output_field=IntegerField(),
)

Comment.objects.filter(post_id=post).annotate(reply_count=subquery)