djadhere/banking/forms.py

from django.forms import ModelForm
from django.core.exceptions import ValidationError

from .models import Payment


class PaymentForm(ModelForm):
    class Meta:
        model = Payment
        fields = ['amount', 'period', 'payment_method', 'start']

    def clean_start(self):
        start = self.cleaned_data['start']
        if self.instance.pk:
            if start < self.instance.start:
                raise ValidationError(_('Invalid value'), code='invalid')
        return start