How to assign default value to Django model Integerfield, while related FormField is optional?

I want to create a Django model Field (IntegerField) with a default value, and also create a form derived from the model, where the field is optional. If it's not set on the form, then when I save the form, I want the default value saved to the DB.

# model.py
class Invoice(models.Model):
    # IntegrityError "Column 'expireDays' cannot be null"
    expireDays = models.PositiveSmallIntegerField(default=1)

    # expireDays = *null* in DB
    expireDays = models.PositiveSmallIntegerField(default=1, blank=True, null=True)

# forms.py
class InvoiceForm(forms.ModelForm):
    # leaving this line out gives invalid form
    expireDays = forms.IntegerField(required=False)
    class Meta:
        model = Invoice

(I used only one of the field declaration lines at a time. :)

I'm not even sure that I'm declaring the default value correctly. The only reference I could find to it was in an article on handling choices by James Bennett. I have yet to find it in the Django docs (I'm using version 1.2 - maybe it's in 1.3?)

Update - I tried setting the field's default value in the MySql database, to no effect. It seems as if, even when the form does not have a value for the field, it goes ahead and assigns null to the DB, over-riding the MySql default value.

Although I am currently just setting a default value in the view that creates the form - I don't really like that, since it puts the responsibility for the field's integrity in the view, not the DB.

The way I would have thought it would work, is that the field could be set, or not, in the form - if set, that value would be written to the DB, and if not set, the DB default would be used. Instead, if not s开发者_C百科et, the form is writing a null to the DB. So what's the point of having a default value in the ModelField declaration if it's not used? What exactly does it do?

i you want field to be optional - just leave second definition in the model and do not add anything in the form definition:

class Invoice(models.Model):
    expireDays = models.PositiveSmallIntegerField(default=1, blank=True, null=True)

class InvoiceForm(forms.ModelForm):
    class Meta:
        model = Invoice

update, so in case there is no value set, use 1 as the field value:

class InvoiceForm(forms.ModelForm):

    def clean_expireDays(self):
        exp_days = self.cleaned_data.get('expireDays')
        if exp_days is None:
            return self.fields['expireDays'].initial
            # above can be: return 1
            # but now it takes value from model definition
        else:
            return exp_days

    class Meta:
        model = Invoice