clean() method in model and field validation

Question

I have a problem with a model's clean() method and basic field validation. here's my model and the clean() method.

class Trial(models.Model):

    trial_start = DurationField()
    movement_start = DurationField()
    trial_stop = DurationField()


    def clean(self):
        from django.core.exceptions import ValidationError
        if not (self.movement_start >= self.trial_start):
            raise ValidationError('movement start must be >= trial start')
        if not (self.trial_stop >= self.movement_start):
            raise ValidationError('trial stop must be >= movement start')
        if not (self.trial_stop > self.trial_start):
            raise ValidationError('trial stop must be > trial start')

My clean() method checks whether certain values are in the correct range. If the user forget to fill out a field, e.g. movement_start, then I get an error:

can't compare datetime.timedelta to NoneType

I'm surprised that I get this error, since the original clean() function should be catching that missing entry (after all movement_start is a required field). So how can I the basic checking for missing values, and my custom check whether values are in certain ranges? Can this be done with model's clean() method, or do I need to use Forms?

EDIT1 to make it more clear: trial_start, movement_start and trial_stop are all required fields. I need to write a clean() method which first checks that all three fields have been filled out, and then, check whether the values are in a certain range.

The following code for example DOES NOT work, since trial_start might be empty. I want to avoid having to check for the existence of each field - django should do that for me.

class TrialForm(ModelForm):

    class Meta:
        model = Trial

    def clean_movement_start(self):
        movement_start = self.cleaned_data["movement_start"]
        trial_start = self.cleaned_data["trial_start"]
        if not (movement_start >= trial_start):
            raise forms.ValidationError('movement start must be >= trial start')
        return self.cleaned_data["movement_start"] 

EDIT2 The reason that I wanted to add this check to the model's clean() method is that objects that are created on the python shell, will automatically be checked for correct values. A form will be fine for views, but I need the value check also for the shell.

Answer 1

I guess that's the way to go:

class TrialForm(ModelForm):

    class Meta:
        model = Trial

    def clean(self):
        data = self.cleaned_data
        if not ('movement_start' in data.keys() and 'trial_start' in data.keys()  and 'trial_stop' in data.keys()):
            raise forms.ValidationError("Please fill out missing fields.")

        trial_start = data['trial_start']
        movement_start = data['movement_start']
        trial_stop = data['trial_stop']

        if not (movement_start >= trial_start):
            raise forms.ValidationError('movement start must be >= trial start')

        if not (trial_stop >= movement_start):
            raise forms.ValidationError('trial stop must be >= movement start')

        if not (trial_stop > trial_start):
            raise forms.ValidationError('trial stop must be > trial start')

        return data

EDIT the downside of this approach is, that value checking will only work if I create objects through the form. Objects that are created on the python shell won't be checked.