Re: MultipleChoiceField form bug?

Hi,

On Dec/22/2020, Benny M wrote:
> That's correct. I am using a class variable since there doesn't seem
> to be a DRY - or at least not messy - way of manipulating this from
> __init__.

It's easy to make this mistake. I was bitten by something similar in a
class based view generating a queryset...

> That said, your explanation makes sense and does answer the "why" of
> this. If there's a way of defining forms inside __init__ I haven't
> found it… which is not to say that it doesn't exist.

Yes, you can

> Here's an example of what I'm referring to: (Pardon my poorly labeled
> model/form names)
>
> ```
> class M2MForm(forms.ModelForm):
> """ Populate/update M2MModel name and m2m references """
> REF_OPTIONS = ((r.id, r.name) for r in RefModel.objects.all())
>
> ref_models = forms.MultipleChoiceField(
> widget=forms.SelectMultiple,
> choices=REF_OPTIONS,
> required=False
> )
>
> class Meta:
> model = M2MModel
> fields = ['name']
> ```


Try something like:

```
class M2MForm(forms.ModelForm):
def __init__(*args, **kwargs):
super().__init__(*args, **kwargs)

# At this point self.fields['name'] already exist...
# ...if you overwriteself.fields['name'] careful
# because you need to save it in save()
#
# self.fields gets created by the __init__() method of
# the parent class (actually grandparent I think :-) )

""" Populate/update M2MModel name and m2m references """
REF_OPTIONS = ((r.id, r.name) for r in RefModel.objects.all())


# this would add the field 'ref_models' in the form, but save()
# will not save. You can overwrite save and save it yourself
self.fields['ref_models'] = forms.MultipleChoiceField(
widget=forms.SelectMultiple,
choices=REF_OPTIONS,
required=False
)

class Meta:
model = M2MModel
fields = ['name']
```

I would look at not using forms.MultipleChoiceField and use
forms.ModelMultipleChoiceField(): then you should be able to pass
queryset (first argument for the ModelMultipleChoiceField) and avoid the
REF_OPTIONS=... line

Let me know what you find :-)

Cheers,

>
> > On Dec 22, 2020, at 9:31 AM, Carles Pina i Estany <carles@pina.cat> wrote:
> >
> >
> >
> > Hi,
> >
> > On Dec/22/2020, Benny M wrote:
> >
> >> I've ran into some unexpected behavior while testing a ModelForm with
> >> a MultipleChoiceField and am trying to determine if this is a bug, or
> >> if maybe I'm doing something out of the ordinary.
> >
> > I think that I might guess what it is. If I could see the code it might
> > be easier.
> >
> > Are you doing something like:
> >
> > class SomeForm(forms.ModelForm):
> > some_field_options = a_dynamic_expression_that_queries_the_DB
> >
> > def __init__(*args, **kwargs):
> > your code
> >
> > So using a class variable instead of an instance variable?
> >
> > Class variables are initialised on "startup" (I'm simplifying) and only
> > once (and might be used via __new__ on ModelFormMetaclass probably, read
> > and validated).
> >
> > Either way, if it's not what I've guessed and you paste some code it
> > might help me / someone else to understand it better.
> >
> > I had once a bug because of using datetime.now() in a class variable
> > when I wanted it the "now" that the view was used not the "now" when the
> > application was started up.
> >
> > --
> > Carles Pina i Estany
> > https://carles.pina.cat
> >
> > --
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>
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Carles Pina i Estany
https://carles.pina.cat

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