Django update rows base on order in queryset - django

Question

I have a simple django model
class Item(Model):
name = CharField()
rank = PositiveIntegerField()
created_at = DateField(auto_now_add=True)
I want to update the object rank based on their order when sorted by a field (name or created_at)
e.g. when ordering by name
[("Pen", 0, "2021-05-04"), ("Ball", 0, "2021-05-04")] => [("Pen", 1, "2021-05-04"), (Ball, 0, "2021-05-04")]
I already know I can do this using bulk_update but it means I have to fetch the objects in memory
items = Items.objects.order_by("name")
for i, item in enumerate(items):
item.rank = i
Item.objects.bulk_update(items, ["rank"])
I was wondering if there is a way to do it with 1 query directly in the database, without having to fetch the data

Answer 1

CREATE TABLE items (
id serial PRIMARY KEY,
name VARCHAR ( 50 ) UNIQUE NOT NULL,
rank smallint
);
insert into items(id, name, rank) values (1, 'A', 0);
insert into items(id, name, rank) values (2, 'B', 0);
insert into items(id, name, rank) values (3, 'C', 0);
select * from items;
id
name
rank
1
A
0
2
B
0
3
C
0
UPDATE items
SET rank=calculated.calc_rank
FROM
(SELECT id AS calc_id,
(ROW_NUMBER() OVER (
ORDER BY name ASC)) AS calc_rank
FROM items) AS calculated
WHERE items.id = calculated.calc_id;
select * from items;
id
name
rank
1
A
1
2
B
2
3
C
3
And perform raw SQL for Django:
sql = '''
UPDATE items
SET rank=calculated.calc_rank
FROM
(SELECT id AS calc_id,
(ROW_NUMBER() OVER (
ORDER BY name ASC)) AS calc_rank
FROM items) AS calculated
WHERE items.id = calculated.calc_id
'''
with connection.cursor() as cursor:
cursor.execute(sql)