Template widget rendering breaks the AdvancedModelIterator pattern

[Jon Dufresne]

Occasionally, when extending ModelChoiceField and ModelMultipleChoiceField with a custom widget, it is useful for the widget to know the object used to create the value, label pair. This could be used to adding custom HTML attributes to very specific choices.

Historically, a pattern was developed to handle this situation named ​AdvancedModelChoiceIterator. The pattern described in the article breaks in 1.11.

Firstly, ChoiceWidget now always expects a 2-tuple returned by ModelChoiceIterator.choice() as it is unpacked in ​ChoiceWidget.optgroups(). The usage is ​documented as requiring a 2-tuple, so maybe this is considered OK.

An alternative I considered was returning a wrapped value to hold the object, hoping this would be accessible from ChoiceWidget.create_option(). Unfortunately this did not work as the value is eagerly coerced to a string very early in ​ChoiceWidget.optgroups(). For example:

class AdvancedChoiceValue:
    def __init__(self, value, obj):
        self.value = value
        self.obj = obj

    def __str__(self):
        return str(self.value)


class AdvancedChoiceIterator(ModelChoiceIterator):
    def choice(self, obj):
        value, label = super().choice(obj)
        return AdvancedChoiceValue(value, obj), label

I think delaying the force_text() until checking the selected value would solve this problem.

[Jon Dufresne]

​PR

[Jon Dufresne]

Added test.

[Jon Dufresne]

I should say that I'm very open to working towards an alternative solution if someone sees a better opportunity to solve the same problem in a more elegant way.

[Tim Graham]

The proposed patch seem sensible given the current APIs. In the long run, it could be nice to consider a change that allows this pattern without so much boilerplate. Any other ideas, Preston?

[Tim Graham <timograham@…>]

In 6d8979f4:

Fixed #27758 -- Reallowed AdvancedModelIterator pattern after template widget rendering.

[Tim Graham <timograham@…>]

In 52e9c1c:

[1.11.x] Fixed #27758 -- Reallowed AdvancedModelIterator pattern after template widget rendering.

Backport of 6d8979f4c2fbfb9fd5db92acd72489cbbcbdd5d1 from master