Automatically resolve Value's output_field for stdlib types.

[Ozan Gerdaneri]

Hi,
I have a model of AModel. AModel has a SearchVectorField named search_vector. I want to update this vector by indexing a string that is not in any other field.

from django.db.models import Value
from django.contrib.postgres.search import SearchVector

AModel.objects.filter(pk=1).update(search_vector=SearchVector(Value("a string to be indexed and inserted to search_vector field")))

This code generates this error:

FieldError: Cannot resolve expression type, unknown output_field

It seemed to be a bug since I found similar usages in forums..

[Simon Charette]

Specifying an explicit output_field for Value should resolve your issue.

value = Value(
    "a string to be indexed and inserted to search_vector field",
    output_field=models.TextField(),
)
AModel.objects.filter(pk=1).update(
    search_vector=SearchVector(value),
)

I guess Value._resolve_output_field could be made smarter for some stdlib types such as str, float, int, Decimal, date, datetime so I'm tentatively accepting on this basis.

[Matt Westcott]

In case it hasn't already been noted - the example above did previously work in 2.2, and the error is a regression in 2.2.1, specifically this commit: ​https://github.com/django/django/commit/88bf635c356b4d3a47e88dc4142b90060ce3c2ef . As such, I think this might justify being upgraded to a bug.

(FWIW, we encountered this independently here: ​https://github.com/wagtail/wagtail/issues/5547)

[Mariusz Felisiak <felisiak.mariusz@…>]

In ea3beb4:

Refs #30446 -- Defined default output_field of text database functions.

This prevented the default behavior of
BaseExpression._resolve_output_field from error'ing out when such
functions accepted both expressions from mixed types
(e.g. SubStr(CharField, IntegerField, IntegerField)).

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1b7623fd:

Refs #30446 -- Defined output_field of BoundingCircle() GIS database function.

This prevented the default behavior of
BaseExpression._resolve_output_field from error'ing out when such
functions accepted both expressions from mixed types
(e.g. BoundingCircle(Polygon, num_seg=12)).

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1e38f11:

Fixed #30446 -- Resolved Value.output_field for stdlib types.

This required implementing a limited form of dynamic dispatch to combine
expressions with numerical output. Refs #26355 should eventually provide
a better interface for that.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 156a2138:

Refs #30446 -- Removed unnecessary Value(..., output_field) in docs and tests.