StringAgg() should set output_field to TextField.

[Swen Kooij]

Example:

class MyModel(models.Model):
     name = models.TextField()

list(MyModel.objects.annotate(names=StringAgg('name', ',')))

Results in the error:

django.core.exceptions.FieldError: Expression contains mixed types: TextField, CharField. You must set output_field.

There are two possible work-arounds:

1. Derive from StringAgg and set the output_field:

class TextStringAgg(StringAgg):
    output_field = TextField()

2. Wrap the expression in ExpressionWrapper:

ExpressionWrapper(StringAgg('name', ','), output_field=TextField())

This seems to be a regression introduced in 3.2 as part of this change: ​https://github.com/django/django/commit/1e38f1191de21b6e96736f58df57dfb851a28c1f

[Mariusz Felisiak]

This side-effect is documented, see 9369f0cebba1f65909a14dec6aa3515ec1eb2557 and #31967. Also, you can set output_field directly in the StringAgg(), e.g.

StringAgg('field', ';', output_field=TextField())

I'm not sure if there is anything specific that we could do.

[Swen Kooij]

Sorry, I didn't quite grasp from that part of the release notes that this is expected.

Setting a default output_field on StringAgg would help. It would restore the original behaviour. It's a bit strange to get this error when dealing with strings. Whether it's a CharField or TextField, you still end up with a string in Python. Having to explicitly add an output_field for StringAgg seems to be just an annoyance.

[Simon Charette]

This will be an issue for all expressions that accept mixed string types.

Since PostgreSQL ​documents that string_agg ( value text, delimiter text ) → text I guess we should default to output_field = models.TextField() to limit this annoyance.

[Mariusz Felisiak]

Replying to Simon Charette:

This will be an issue for all expressions that accept mixed string types.

Since PostgreSQL ​documents that string_agg ( value text, delimiter text ) → text I guess we should default to output_field = models.TextField() to limit this annoyance.

OK, agreed. Swen, would you like to prepare a patch (with tests)?

[ali sayyah]

Should I make the patch for 3.2 or the main branch?

[Claude Paroz]

main branch (which will target 4.1, now that 4.0 is feature frozen).

[ali sayyah]

Replying to Claude Paroz:

main branch (which will target 4.1, now that 4.0 is feature frozen).

Thank you.

[ali sayyah]

PR with the fix and a unittest: ​https://github.com/django/django/pull/14898

[Mariusz Felisiak <felisiak.mariusz@…>]

In ca583783:

Fixed #33114 -- Defined default output_field of StringAgg.

Thanks Simon Charette for the review.