Document that ModelMultipleChoiceField queryset may be None

[wraus]

I have a ModelMultipleChoiceField whose queryset I want to populate after the initialization of the form.

class FooMultipleChoiceForm(forms.Form)
    foo_select = forms.ModelMultipleChoiceField(
        label="Foo Selection",
        help_text="Select a Foo from the choices above.",
        queryset=None,
    )

    def __init__(self, bar, *args, **kwargs):
        super(FooMultipleChoiceForm, self).__init__(*args, **kwargs)
        self.fields['foo_select'].queryset = bar.foo_set.all()

Currently, there are two major issues with this:

1. Even if I want to set the queryset later in __init__ with self.fields['foo_select'], I still have to specify a queryset attribute when declaring the field, even though the given queryset will be overwritten in __init__. This not only seems redundant, but it feels odd to be forced to enter a value for the explicit purpose of having it overwritten later, especially when providing None for the queryset attribute doesn't cause any issues if a new queryset is provided in the __init__ of the form.
2. If I omit the queryset attribute in ModelMultipleChoiceField, I get a confusing error message:

   TypeError: __init__() takes at least 2 arguments (3 given)
   

   Nothing is mentioned about the missing queryset argument, and given that I'm setting it later, it's easy to overlook that it's the cause of the problem.

Given that setting queryset to None doesn't cause problems as long as the queryset is initialized later in the form, it seems like it might make sense to allow queryset to be unspecified at field declaration time, as long as it's specified later.

[Tim Graham]

I have used this pattern before as well, however, I think it would be best to document that queryset may be None if you want to specify it later. Making it optional seems likely to trip up newbies.

[Tim Graham <timograham@…>]

In ced3e303ca6c902c14d08b68806230ce04c71ff7:

Fixed #23250 -- Documented that ModelMultipleChoiceField queryset may be None.

[Tim Graham <timograham@…>]

In 8e77ac634ffcbeab634aa8c3f1bda7e6fab229a0:

[1.6.x] Fixed #23250 -- Documented that ModelMultipleChoiceField queryset may be None.

Backport of ced3e303ca from master

[Tim Graham <timograham@…>]

In 1b89f976f4919a36c6b70eba428c3a02a94e2822:

[1.7.x] Fixed #23250 -- Documented that ModelMultipleChoiceField queryset may be None.

Backport of ced3e303ca from master