QuerySet.annotate() crashes when grouping by OuterRef().

[Oliver Ford]

I'm trying to construct a query like:

cheapest_query = models.Subquery(qs.annotate(
    groupbytype=models.OuterRef("type"),
).values("groupbytype")).annotate(
    cheapest=models.Min("price"),
).values("cheapest"))

qs = qs.annotate(premium_over_cheapest=models.F("price") - cheapest_query)

However this fails in the second annotate (cheapest=) since ResolvedOuterRef does not descend from BaseExpression; so it has no get_group_by_cols method:

   File "/usr/local/lib/python3.8/site-packages/django/db/models/query.py", line 1078, in annotate
     clone.query.set_group_by()
  File "/usr/local/lib/python3.8/site-packages/django/db/models/sql/query.py", line 1938, in set_group_by
     inspect.getcallargs(annotation.get_group_by_cols, alias=alias)
 AttributeError: 'ResolvedOuterRef' object has no attribute 'get_group_by_cols'

​https://github.com/django/django/blob/master/django/db/models/expressions.py

[Oliver Ford]

Perhaps I was doing something silly in the first place; I've resolved my issue by changing the subquery to first annotate (group by) all the types, whatever they are, not OuterRef, and then only after the Min annotation filtering on the OuterRef("type") matching the grouped by type.

I'll leave this open for now though in case it's desired to improve the error message in this case, or perhaps there's a more sensibly-intentioned query that does produce the same error.

[Mariusz Felisiak]

Grouping by OuterRef() doesn't have much value, it's like using a constant value, so IMO we can easily fix this with:

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 2e831b21ac..a165923784 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -562,6 +562,9 @@ class ResolvedOuterRef(F):
     def relabeled_clone(self, relabels):
         return self
 
+    def get_group_by_cols(self, alias=None):
+        return []
+
 
 class OuterRef(F):
     def resolve_expression(self, *args, **kwargs):

[Oliver Ford]

Why not the column that OuterRef is referencing? That was my intended behaviour, though I eventually realised I was overcomplicating it and just needed to group on the column directly, not an outer ref, and filter on the outer ref.

[Mariusz Felisiak]

Oliver, OuterRef() is a field from the outer query, so it will be the same for all rows in a subquery. Is there any value in adding it to a GROUP BY clause? e.g.

(SELECT MIN("price") from "subquery_table" GROUP BY "outer_table"."type")

is equivalent to

(SELECT MIN("price") from "subquery_table")

[Yuri Savin]

I think I overestimated my strength with this task

[Rohit Jha]

PR: ​https://github.com/django/django/pull/12489

[Rohit Jha]

Tests => ​​https://github.com/django/django/pull/12489

[Mariusz Felisiak <felisiak.mariusz@…>]

In 486786c:

Fixed #31251 -- Disabled grouping by OuterRef() annotation.