Django validation calling clean() even when required value missing

Question

I am new to Django and somewhat confused regarding the validation steps during form processing. I am aware that all form field types (in my case a ModelForm) are required by default. I assumed that Django would raise a VaidationError in case a required form field was left blank without calling the form's clean method.

This is why I did not check if any data was set in the following clean() method:

def clean(self):
    date =  self.cleaned_data.get('date')
    time_start = self.cleaned_data.get('time_start')
    time_end = self.cleaned_data.get('time_end')
    user_type = self.cleaned_data.get('user_type')

    if Event.objects.filter(user_type=user_type, date=date, 
                            time_start__lt=time_start, 
                            time_end__gt=time_start).exclude(pk=self.instance.pk).count():
        raise forms.ValidationError("Overlapping with another event.")

Submitting the form while leaving all fields blank causes a

ValueError: Cannot use None as a query value.

If I remove my clean() method I will get the expected ValidationErrors for not filling out the required fields - which is what I expected with the clean() method still in place.

Any idea what could cause this to happen? I would be surprised if Django does not check for required values before it calls clean.

Answer 1

This is strange because the validation of the fields is performed before the calling of the form's clean method. Moreover an error raised from a field is stored in form.my_field.errors while the errors returned from the form's clean method are accumulated in form.non_field_errors.

Below is the order of the validations performed in a form:

full_clean()
    |
Field clean() [field's built-in clean method]
    |
Form clean_*() [custom validation method for field]
    |
Form clean() [form's clean method]
    |
cleaned_data/errors