Django - how to force user to select a certain amount of options in a checbox modelform?

Question

I'm making a simple survey built around a modelform:

models.py

class Fruits(models.Model):
     author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
     title = models.CharField(max_length=200, default='')
     text = models.TextField(default='')
     banana = models.BooleanField(default=False)
     grape = models.BooleanField(default=False)
     apple = models.BooleanField(default=False)
     mango = models.BooleanField(default=False)

forms.py

class FruitPicker(ModelForm):
    class Meta:
        model = Fruits
        fields = [
        'title',
        'text',
        'banana',
        'grape',
        'apple',
        'mango'
    ]
    
    widgets = {
        'text': forms.Textarea(attrs={"style": "height:10em;" "width:60em;"}),
        'banana': forms.CheckboxInput(attrs={"style": "margin-left:350px;"}),
        'grape': forms.CheckboxInput(attrs={"style": "margin-left:350px;"}),
        'apple': forms.CheckboxInput(attrs={"style": "margin-left:350px;"}),
        'mango': forms.CheckboxInput(attrs={"style": "margin-left:350px;"})
    }

Now let's say I want to make sure the user has to select a minimum amount of 2 fruits, but a maximum amount of 3. How would I go about doing this on the back-end?

If you want to do it with regular form validation in the backend, you can do it in the `clean` method. If you want to do it in the frontend, you're probably going to have to do a bit of custom JS. — michjnich, Oct 27 '21 at 13:39
Back-end would be ok - I edited my question to clarify this now. — LarsLill, Oct 27 '21 at 13:56
Willem beat me to the answer then - you can just use the clean method :) — michjnich, Oct 27 '21 at 14:14

score 1 · Accepted Answer · answered Oct 27 '21 at 14:11

You can override the clean method of the form to check if between two and three BooleanFields are checked:

from django.core.exceptions import ValidationError

class FruitPicker(ModelForm):
     # &vellip;

    def clean(self, *args, **kwargs):
        data = super().clean(*args, **kwargs)
        total = sum([data[x] for x in ['banana', 'grape', 'apple', 'mango']])
        if not (2 <= total <= 3):
            raise ValidationError('You should select between 2 and 3 fruits.')
        return data

Here the sum(…) sums up the booleans and uses 0 for False, and 1 for True, it thus counts the number of Trues. If that number is not between 2 and 3, then it raises a ValidationError.

This worked! For some reason I had to remove the *args and **kwargs behind super().clean though. — LarsLill, Oct 27 '21 at 17:35

Django - how to force user to select a certain amount of options in a checbox modelform?

1 Answers1