How to do "choices" from DB in form field without breaking migrations and such

[jason...@gmail.com]

Hello everyone,

I have a form field that is pulling choices from the database:

class FooJBossForm(Form):
    biz_service = MultipleChoiceField(
        choices=BizService.objects.filter(
            biz_unit__bu_name='Foo',
        ).order_by('bs_name').values_list('bs_name', 'bs_name').distinct(),
        required=False,
        label='Business Service',
        widget=SelectMultiple(attrs={'class': 'form-control'}),
    )

    ...

This worked great until someone else on my team attempted to start with a fresh DB and was getting "relation does not exist" errors trying to do the initial import.

Commenting out these "choices=" parameters in `forms.py` fixes the issue.  What's the right way to go about populating form field choices from the DB at access time?

Thanks,

j

[jason...@gmail.com]

I should have included that the reason I'm using a `MultipleChoiceField` instead of a `ModelMultipleChoiceField` here is because I want the POSTed variable to be the business service name (string) not the primary key of the selected record.

j

[C. Kirby]

You can set the choices in the form __init__ to handle the issue:

 class FooJBossForm(Form):
    biz_service = MultipleChoiceField(

        choices=[],

        required=False,
        label='Business Service',
        widget=SelectMultiple(attrs={'class': 'form-control'}),
    )

    def __init__(self, *args, **kwargs):
        super(FooJBossForm, self).__init__(*args, **kwargs)
        try:
            self.fields['biz_service'].choices = BizService.objects.filter(
                                   biz_unit__bu_name='Foo',
                                                 ).order_by('bs_name')\
                                                  .values_list('bs_name', 'bs_name')\
                                                  .distinct()
        except ProgrammingError:
            pass #OR Some placeholder choices

Kirby

[jason...@gmail.com]

Thank you very much.  This worked perfectly.  Consequently it also fixed my issue of those select boxes not reflecting changes in the list until the web server was restarted since the data is now being pulled at instantiation instead of in the class definition.

j

[C Kirby]

Yeah, I meant to write about that. If you put a database filter in a field definition (model or form) it only runs it on start/restart. Using the method I put, or a callback function in the choices, allows the choices to reflect reality of the data.

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