由于缺少值，因此ModelForm无法验证，但是模型字段为null = True

Question

I have a problem where my ModelForm is trying to assign '' to a field (it saves fine if I actually provide the primary key of the Product , but it's not a compulsory field, and won't save if the field is left blank). I take it that the ORM it's trying to set that field to '' but:

• Shouldn't '' be coerced to None , and;
• Why isn't the model form trying to set that field to None in the first place instead of '' ?

models.py

class Question(models.model):
    fk_product = models.ForeignKey(Product, on_delete=models.SET_NULL, null=True, related_name="product_question")

forms.py

class QuestionForm(forms.ModelForm):
    fk_product=forms.ChoiceField(required=False)
    class Meta:
        model = Question
        fields = ['fk_product',]

The error:

Cannot assign "''": "Question.fk_product" must be a "Product" instance.

The view code that produces the error:

    QuestionModelFormset = modelformset_factory(Question,
                                form=QuestionForm,
                                extra=1)
    question_formset = QuestionModelFormset(
                            data=request.POST,
                            files=request.FILES,
                            queryset=Question.objects.all())
    if not question_formset.is_valid(): #error occurs on this line

Answer 1

Try adding blank=True too.

null=True means that this field is allowed to be NULL in database.

blank=True means it can be submitted without a value in forms. Otherwise it must have value.