Cannot assign expressions to spatial fields

[Jacob Walls]

SpatialProxy is too strict in the types it allows to be assigned to spatial fields -- it should allow expressions (if it can be implemented simply enough).

See ​fiddle:

from django.contrib.gis.db.models import PointField
from django.db import models

class Employee(models.Model):
    home = PointField()
    work = PointField()

def run():
    instance = Employee.objects.create(
        home="POINT(1 1)", work="POINT(2 2)"
    )
    # Work from home.
    instance.work = models.F("home")
    instance.save()

  File "/usr/local/lib/python3.12/site-packages/django/contrib/gis/db/models/proxy.py", line 82, in __set__
    raise TypeError(
TypeError: Cannot set Employee SpatialProxy (POINT) with value of type: <class 'django.db.models.expressions.F'>

---

Marking new feature, as we should document that we added support.

[Md. Saikat Islam]

I was able to reproduce the reported TypeError with F("home") assigned to a PointField. I'll work on a fix to allow expressions to be assigned to spatial fields.

[Md. Saikat Islam]

The issue persists with Func, Value, and other Combinable subclasses. So it's not only F(django.db.models.expressions.Func) that's an issue. The fix would be to check whether the value's type is Combinable in __set__ in proxy.py.

[Md. Saikat Islam]

My PR is ready for review: ​https://github.com/django/django/pull/21893