Prevent inappropriate order-based grouping on values+annotate queries

[Martin Chase]

class Foo(models.Model):
    name = models.TextField()
    type = models.TextField()

    class Meta:
        ordering = ["name",]

Foo.objects.all().values("type").annotate(models.Count("name"))

The above code fails to generate the expected result without adding .order_by() to the query set.

Ticket #10574 called this fixed with documentation, but indicated a proper fix would be preferred once backwards compatibility wasn't an issue. Lest this fall off the radar for being closed, I am putting this ticket in for 2.0.

[Martin Chase]

possible fallback orderings:

• pk
• [the ordering fields] & [the fields referenced in the .values]

[Martin Chase]

(that was a set & operator. sorry for the confusing notation.)

possible fallback orderings:

• pk
• only those fields which are in both the ordering and the values
• all fields in the ordering which are not also in an annotate

[Martin Chase]

attached unordering.patch is a poor attempt at changing the ordering.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

Simon, do you have any opinions about this change given we abandoned the idea of "Django 2.0" with large backwards incompatible changes? Not sure if a deprecation path to change the behavior is feasible.

[Simon Charette]

I think we could and should make this change as it really common pitfall but it should follow a deprecation path.

I've personally stopped using ordering because of its unexpected interaction with distinct() and values() for grouping by.

The deprecation path should detect when values() is used for grouping on a queryset where the only defined ordering is from _meta.ordering in the SQL compiling step and raise a deprecation warning saying such ordering will be ignored in future version of Django.

[Simon Charette]

I think we should go as far as requiring all ordering clauses to be explicitly specified through values() when used to group by.

For example, in the following case

Foo.objects.order_by('name').values('type').annotate(Count('name'))

A deprecation warning would be raised to notify the user that 'name' won't automatically be added to values() in future Django version where the previous queryset would be equivalent to the following:

Foo.objects.values('type').annotate(Count('name'))

and the current behaviour could be preserved by adding 'name' to values():

Foo.objects.order_by('name').values('type', 'name').annotate(Count('name'))

I haven't tested it myself to see if it's already the case but we should also disallow ordering by a non-grouped column or aggregate alias.

Foo.objects.values('type').annotate(Count('name')).order_by('name') should raise a value error on the order_by('name') call.

[Mariusz Felisiak]

I totally agree with Simon. In my opinion we shouldn't automatically add model.Meta.ordering to values(), values_list() and distinct(). I remember that a few years ago I lost a lot of time when I tried to figure out why the result wasn't what I expected.

[Tim Graham]

I started a ​PR. It's not complete or ready for review yet.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 1b1f64ee:

Refs #14357 -- Deprecated Meta.ordering affecting GROUP BY queries.

Thanks Ramiro Morales for contributing to the patch.

[Carlton Gibson]

I'm unchecking Has patch, since the PR (deprecating ordering being used for grouped queries was merged). What remains to close this ticket?

[Tim Graham]

The behavior change will happen in Django 3.1 when the deprecation ends.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0ddb4ebf:

Refs #14357 -- Made Meta.ordering not affect GROUP BY queries.

Per deprecation timeline.

[Yuri Konotopov]

I beleave this was not properly fixed in commit 0ddb4ebf.
While commit 0ddb4ebf removes ORDER BY when Meta.ordering is used it still does populates GROUP BY with Meta.ordering fields thus leads to wrong aggregation.

Look to get_group_by() at compiler.py [1] - it populates GROUP BY using previously obtained order_by with Meta.ordering fields [2][3].
For what I see we must filter off Meta.ordering fields from order_by in get_group_by() function.

[1] ​https://github.com/django/django/blob/3.1.7/django/db/models/sql/compiler.py#L128
[2] ​https://github.com/django/django/blob/3.1.7/django/db/models/sql/compiler.py#L56
[3] ​https://github.com/django/django/blob/3.1.7/django/db/models/sql/compiler.py#L286

[Mariusz Felisiak]

Please don't reopen closed tickets. You can create a new ticket if you can provide a sample project or test and prove that Meta.ordering is still included.

[GitHub <noreply@…>]

In fcd44b88:

Refs #14357 -- Updated docs about interaction between aggregations and QuerySet.order_by().

Obsolete since 0ddb4ebf7bfcc4730c80a772dd146a49ef6895f6.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 9b096063:

[3.2.x] Refs #14357 -- Updated docs about interaction between aggregations and QuerySet.order_by().

Obsolete since 0ddb4ebf7bfcc4730c80a772dd146a49ef6895f6.
Backport of fcd44b889f36c4be87910745614a0a4c88d7a3d8 from main

[Adam Sołtysik]

For anyone surprised, just like me, that Meta.ordering still breaks aggregation queries in Django 3.1 and 3.2, despite what the documentation says ​here and ​here, it seems that this was finally fixed in #32546 in Django 4.0. Possibly the documentation should be updated to reflect the actual version when the change was implemented, as there seems to be nothing about it in the ​release notes for 4.0. Or maybe the change should be backported to reflect the documentation.