Allow model field choices to accept callables.

[Christopher Grebs]

Django 1.8 introduced the ability to pass a callable for 'choices' but it seems the check-framework was not adapted to this.

Pull Request: ​https://github.com/django/django/pull/4434/

[Tim Graham]

It was added for django.forms.ChoiceField, but not model field choices. It might be a reasonable feature request, but it needs to be implemented. It might cause some difficulty in migrations, but I will accept the ticket if someone wants to investigate the possibility.

[Christopher Grebs]

Oh, then I misread the feature. I'll try to update the patch to port this to the model field choices as well.

[Mariusz Felisiak]

See also discussion in #28033.

[Natalia Bidart]

While reviewing/working on #31262, we noticed that the choices refactoring that is being proposed in ​PR #16943 will help considerably to this ticket. Besides what is discussed in that PR, the following diff should allow model field's callable choices to be serialized as functions (similar to callable default or form ChoiceField's choices).

django/db/migrations/serializer.py

@@ -15 +15 @@
 from django.db import models
 from django.db.migrations.operations.base import Operation
 from django.db.migrations.utils import COMPILED_REGEX_TYPE, RegexObject
+from django.utils.choices import CallableChoiceIterator
 from django.utils.functional import LazyObject, Promise
 from django.utils.version import PY311, get_docs_version
 
@@ -329 +330 @@
         return "uuid.%s" % repr(self.value), {"import uuid"}
 
 
+class CallableChoiceIteratorSerializer(BaseSerializer):
+
+    def serialize(self):
+        return FunctionTypeSerializer(self.value.choices_func).serialize()
+
+
 class Serializer:
     _registry = {
         # Some of these are order-dependent.
@@ -351 +358 @@
             types.BuiltinFunctionType,
             types.MethodType,
         ): FunctionTypeSerializer,
+        CallableChoiceIterator: CallableChoiceIteratorSerializer,
         collections.abc.Iterable: IterableSerializer,
         (COMPILED_REGEX_TYPE, RegexObject): RegexSerializer,
         uuid.UUID: UUIDSerializer,

django/db/models/fields/__init__.py

@@ -315 +315 @@
         if not self.choices:
             return []
 
-        if not is_iterable(self.choices) or isinstance(
-            self.choices, (str, CallableChoiceIterator)
-        ):
+        if not is_iterable(self.choices) or isinstance(self.choices, str):
             return [
                 checks.Error(
                     "'choices' must be a mapping (e.g. a dictionary) or an iterable "

[Natalia Bidart]

​PR

[Natalia Bidart]

Setting as patch needs improvement because I'd like to cover a few use cases with the community first.

[Natalia Bidart]

​Forum Post

[Natalia <124304+nessita@…>]

In 691f70c:

Fixed #24561 -- Added support for callables on model fields' choices.