Formset Validation

Introduction

Customize formset validation beyond individual form validation.

Key Concepts

• BaseFormSet.clean(): Cross-form validation logic.
• min_num/max_num: Form count limits with validate_min/validate_max.
• self.forms: List of form instances.

Real World Context

A quiz builder requiring 5-50 unique questions. The formset clean() checks duplicates while min_num enforces minimums.

Deep Dive

Custom Formset Class

python
from django.forms import BaseFormSet

class BaseBookFormSet(BaseFormSet):
    def clean(self):
        if any(self.errors):
            return  # Skip if form errors exist
        
        titles = []
        for form in self.forms:
            if form.cleaned_data:
                title = form.cleaned_data.get('title')
                if title in titles:
                    raise ValidationError('Duplicate titles not allowed')
                titles.append(title)

BookFormSet = formset_factory(
    BookForm,
    formset=BaseBookFormSet,
    extra=3
)

Min/Max Forms

python
BookFormSet = formset_factory(
    BookForm,
    min_num=1,      # Minimum forms required
    max_num=10,     # Maximum forms allowed
    validate_min=True,
    validate_max=True,
)

Common Pitfalls

1. Not checking self.errors first -- Always start with if any(self.errors): return.
2. Validating deleted forms -- Skip forms with DELETE flag.

Best Practices

1. Combine min_num with validate_min -- min_num alone is display-only.
2. Specific error messages -- "Duplicate titles" beats "Invalid formset".

Summary

Override clean() in BaseFormSet for cross-form validation. Use min_num/max_num to enforce limits.