How can I implement custom validation in Django model forms?

How can I implement custom validation in Django model forms?

You can implement custom validation in Django model forms by overriding the clean method or using field-specific validators.

Can you show me an example of overriding the clean method?

Sure! Here's a simple example:

from django import forms

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = ['my_field']

    def clean_my_field(self):
        data = self.cleaned_data['my_field']
        if data < 0:
            raise forms.ValidationError('Value cannot be negative!')
        return data

What if I want to validate multiple fields together?

In that case, you can override the clean method of the form itself:

def clean(self):
    cleaned_data = super().clean()
    field1 = cleaned_data.get('field1')
    field2 = cleaned_data.get('field2')
    if field1 and field2 and field1 > field2:
        raise forms.ValidationError('Field1 must be less than Field2!')