Can't aggregate annotations with array parameters

[Daniel Keller]

If I use a database function that takes an array parameter, for example a histogram:

import math
from django.db import models
from django.contrib.postgres.fields import ArrayField

class Log2Bucket(models.Func):
    function = 'width_bucket'

    def __init__(self, expression, a, b, **extra):
        max_exp = math.ceil(math.log2(b / a))
        buckets = [a * 2**exp for exp in range(max_exp + 1)]
        buckets = models.Value(buckets, output_field=ArrayField(models.FloatField())) ###
        super().__init__(expression, buckets,
            output_field=models.PositiveSmallIntegerField(), **extra)

and a simple model

class Foo(models.Model):
    bar = models.FloatField()

It works fine in queries like

Foo.objects.annotate(b=Log2Bucket('bar', 1, 100)).values('b')

but for

Foo.objects.annotate(b=Log2Bucket('bar', 1, 100)).values('b').annotate(count=models.Count('pk'))

I get

...
  File "/home/daniel_keller/shared/raumplus_2/env/lib/python3.4/site-packages/django/db/models/sql/compiler.py", line 128, in get_group_by
    if (sql, tuple(params)) not in seen:
TypeError: unhashable type: 'list'

referring to the list passed to Value.

Honestly, I'm not sure if this counts as a bug or a new feature, since it's clearly unintended behavior, but none of Django's database functions need it to work.

I can work around it by replacing the marked line with

buckets = Value("{%s}" % ','.join(map(str, buckets))) ###

but this is IMO very ugly.

[Tim Graham]

I'm not sure what the proper resolution is either, however, I see the ​documentation for Value says, "When you need to represent the value of an integer, boolean, or string within an expression, you can wrap that value within a Value()." You're passing a list to Value which contradicts the documentation -- perhaps a more helpful message could be raised.

[Josh Smeaton]

Specifically, Value can represent types that the database driver (such as psycopg2) has adapters for. But the bug you're hitting is that Value represents itself as a a SQL parameter, and is then hashed to see if it already exists.

Can you try ExpressionList ​https://github.com/django/django/blob/master/django/db/models/expressions.py#L774 and see if that works for your case? It might be worth documenting ExpressionList beside Value for these cases if it works.

[Tomer Chachamu]

It seems like we just need to make the parameters to cur.execute always be hashable. So in this case the list [1, 2, 4, 8, 16, 32, 64, 128] returned from get_db_prep_value. I've added a patch.

[Tim Graham]

Tests are needed to demonstrate what's fixed.

Most likely the patch doesn't qualify for a backport to 1.11 based on our ​supported versions policy (it would have to be a regression from an older version of Django) so you can remove the release note in the PR.

[Tim Graham]

#28767 may be related if not a duplicate.

[Tim Graham]

I left comments for improvement on the PR.

[Tim Graham]

#29139 is the same exception but a different cause involving aggregating JSONField with KeyTransform. The proposed patch doesn't fix that issue (not that is has to).

[Mariusz Felisiak <felisiak.mariusz@…>]

In ceab25bc:

Refs #28762 -- Added test for aggregating over a function with ArrayField parameters.

Fixed in d87bd29c4f8dfcdf3f4a4eb8340e6770a2416fe3.

[Mariusz Felisiak]

Fixed in d87bd29c4f8dfcdf3f4a4eb8340e6770a2416fe3.