Thanks, but this solution doesn't seem acceptable to me, as this means we essentially do brute-force database queries for every single unique field that exists on the model until we find a match.

An ideal solution is to change the original logic from querying based on pk to instead query with the ForeignKey's to_field directly. It might be tricky because we don't have a reference to the model field (which has the to_field) nor the form field (which has the to_field_name) within the widget class.

We might be able to update the BaseChooser's init signature to also accept a to_field_name, then pass the model field's to_field as the value using a callable that gets registered by Wagtail. See the following example on how we do that to set the target_models for AdminPageChooser:

We probably need to do it in the ChooserViewSet's form field override registration:

Then, we'll be able to do something like

self.model_class.objects.get(**{self.to_field_name: value})

Thanks @laymonage for the feedback, makes perfect sense, I used this approach for minimal changes only to avoid the crash.
I'll update this by keeping in mind your suggestions.

This test only works because we are brute forcing the problem, but we're not actually testing the integration with ForeignKey's to_field mechanism.

Would it work if we just add this to the above list of variable names?

Could you explain why this is necessary? Inspecting the code object (__code__) is something we've never done in Wagtail and likely never will without a good reason, and having to copy over the chooser attributes in here seems like a sign that we should go for a different approach.

If the problem is because SnippetChooserViewSet.widget_class actually returns an instance instead of a class:

that means we probably should refactor that to return a class.

The base ChooserViewSet.widget_class already creates the class dynamically with type() so it shouldn't be a stretch to do the same for SnippetViewSet:

e.g.

base_widget_class = AdminSnippetChooser

@cached_property 
def widget_class(self): 
    return type(
        f"{self.model.__class__.__name__}Chooser",
        (self.base_widget_class,),
        {"model": self.model, "icon": self.icon},
    )

I was expecting the form field override to then be:

            register_form_field_override(
                ForeignKey,
                to=self.model,
                override=lambda db_field: {
                    "widget": self.widget_class(to_field_name=db_field.remote_field.field_name)
                },
            )