How to handle the validation of the model form when the model has a clean method if the model form excluded some fields?

Question

I have this model:

class IeltsExam(Model):

    student = OneToOneField(Student, on_delete=CASCADE)
    has_taken_exam = BooleanField(default=False,)
    listening = FloatField(choices=SCORE_CHOICES, null=True, blank=True, )
    reading = FloatField(choices=SCORE_CHOICES, null=True, blank=True, )
    exam_date = DateField(null=True, blank=True, )

    non_empty_fields = \
        {
            'listening': 'please enter your listening score',
            'reading': 'please enter your reading score',
            'exam_date': 'please specify your exam date',
        }

    def clean(self):
        errors = {}
        if self.has_taken_exam:
            for field_name, field_error in self.non_empty_fields.items():
                if getattr(self, field_name) is None:
                    errors[field_name] = field_error
        if errors:
            raise ValidationError(errors)

and have this modelform

class IeltsExamForm(ModelForm):

    class Meta:
        model = IeltsExam
        fields = ('has_taken_exam', 'listening', 'reading', )

when I submit this form in template, I get the below error:

ValueError at /
'ExamForm' has no field named 'exam_date'.

and

During handling of the above exception ({'listening': ['please enter your listening score'], 'reading': ['please enter your reading score'], 'exam_date': ['please specify your exam date']}), another exception occurred:

The error happens in my view where I am validating the form. My database logic is such that I need to have an exam_date field and it should be mandatory to fill if has_taken_exam is checked. However, in ExamForm, for business reasons, I do not need the exam_date. How can I tell ExamForm to turn a blind eye to the exam_date, as I am not saving the modelform instance?

Answer 1

Perform the validation on model's save()

Consider the following model:

class Exam(Model):

    student = OneToOneField(Student, on_delete=CASCADE)
    has_taken_exam = BooleanField(default=False)
    score = FloatField(choices=SCORE_CHOICES, null=True, blank=True)
    exam_date = DateField(null=True, blank=True)

    def save(self, *a, **kw):
        if self.has_taken_exam and not self.exam_date:
            raise ValidationError("Exam date must be set when has_taken_exam is True")
        return super().save()

Answer 2

After the ModelForm is initialised, it has an instance attribute which is the model instance on which clean() will be called. So if you remove exam_date from the instance's non_empty_fields dictionary, it won't use it in clean:

class IeltsExamForm(ModelForm): 
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.instance.non_empty_fields.pop('exam_date')

And you could do that for each field in self._meta.exclude.

However, when doing that, the attribute non_empty_fields should not be a class attribute but an instance property. Modifying the instance's non_empty_fields actually modifies the class attribute (it's a dictionary so it's mutable), which will have unintended side-effects (once removed, it's removed for any subsequent instance you create). Change your model to set the attribute in the init method:

class IeltsExam(Model):
    # ...
    # remove the class attribute non_empty_fields

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.non_empty_fields = { ... }

In general, I would advise you to only use a ModelForm if you're actually going to save the model, in which case a class attribute is the cleaner approach. Instead of doing all this, if your form isn't going to save the actual model, you should not use a ModelForm but a Form and define all fields and cleaning in the form itself.