Django ORM: window function with subsequent filtering
Question:
Answering this question, I found out that window functions are not allowed to combine with filter (technically, they are, but filter clause affects the window). There is a hint to wrap window function in an inner query, so that final SQL looks like this (as I understand):
SELECT * FROM (
SELECT *, *window_function* FROM TABLE)
WHERE *filtering_conditions*
The question is: how can I write this query with Django ORM?
Answers:
There are developers interested in solving it but it’s not something possible with the ORM right now.
One proposed solution would be to add a QuerySet.subquery()
or .wrap()
method that pushes the queryset within a subquery so it can then be filtered.
Update, Django 4.2 will support it.
You need to use raw query. In order to do multiple queries at one. for further information django documentation
for p in Person.objects.raw('''
SELECT * FROM (SELECT *, *window_function* FROM TABLE)
WHERE *filtering_conditions*'''):
print(p)
# John Smith
# Jane Jones
Other thing you can do is the following.
model.py
class Category(models.Model):
name = models.CharField(max_length=100)
class Hero(models.Model):
# ...
name = models.CharField(max_length=100)
category = models.ForeignKey(Category, on_delete=models.CASCADE)
benevolence_factor = models.PositiveSmallIntegerField(
help_text="How benevolent this hero is?",
default=50
)
querySet.py
hero_qs = Hero.objects.filter(category=OuterRef("pk"))
.order_by("-benevolence_factor")
Category.objects.all()
.annotate(most_benevolent_hero=Subquery(hero_qs.values('name')[:1]))
Generated SQL would look like this..
SELECT "entities_category"."id",
"entities_category"."name",
(SELECT U0."name"
FROM "entities_hero" U0
WHERE U0."category_id" = ("entities_category"."id")
ORDER BY U0."benevolence_factor" DESC
LIMIT 1) AS "most_benevolent_hero"
FROM "entities_category"
Another solution is Common Table Expressions (CTE), and with the help of django-cte, you could achieve what you want:
cte = With(
YouModel.objects.annotate(
your_window_function=Window(...),
)
)
qs = cte.queryset().with_cte(cte).filter(your_window_function='something')
Which translates roughly to:
WITH cte as (
SELECT *, WINDOW(...) as your_window_function
FROM yourmodel
)
SELECT *
FROM cte
WHERE cte.your_window_function = 'something'
Answering this question, I found out that window functions are not allowed to combine with filter (technically, they are, but filter clause affects the window). There is a hint to wrap window function in an inner query, so that final SQL looks like this (as I understand):
SELECT * FROM (
SELECT *, *window_function* FROM TABLE)
WHERE *filtering_conditions*
The question is: how can I write this query with Django ORM?
There are developers interested in solving it but it’s not something possible with the ORM right now.
One proposed solution would be to add a QuerySet.subquery()
or .wrap()
method that pushes the queryset within a subquery so it can then be filtered.
Update, Django 4.2 will support it.
You need to use raw query. In order to do multiple queries at one. for further information django documentation
for p in Person.objects.raw('''
SELECT * FROM (SELECT *, *window_function* FROM TABLE)
WHERE *filtering_conditions*'''):
print(p)
# John Smith
# Jane Jones
Other thing you can do is the following.
model.py
class Category(models.Model):
name = models.CharField(max_length=100)
class Hero(models.Model):
# ...
name = models.CharField(max_length=100)
category = models.ForeignKey(Category, on_delete=models.CASCADE)
benevolence_factor = models.PositiveSmallIntegerField(
help_text="How benevolent this hero is?",
default=50
)
querySet.py
hero_qs = Hero.objects.filter(category=OuterRef("pk"))
.order_by("-benevolence_factor")
Category.objects.all()
.annotate(most_benevolent_hero=Subquery(hero_qs.values('name')[:1]))
Generated SQL would look like this..
SELECT "entities_category"."id",
"entities_category"."name",
(SELECT U0."name"
FROM "entities_hero" U0
WHERE U0."category_id" = ("entities_category"."id")
ORDER BY U0."benevolence_factor" DESC
LIMIT 1) AS "most_benevolent_hero"
FROM "entities_category"
Another solution is Common Table Expressions (CTE), and with the help of django-cte, you could achieve what you want:
cte = With(
YouModel.objects.annotate(
your_window_function=Window(...),
)
)
qs = cte.queryset().with_cte(cte).filter(your_window_function='something')
Which translates roughly to:
WITH cte as (
SELECT *, WINDOW(...) as your_window_function
FROM yourmodel
)
SELECT *
FROM cte
WHERE cte.your_window_function = 'something'