Django Validation: Models vs Forms

This is a great question about Django architecture and validation placement. Let me provide some guidance on this topic.

Keep Model Validation in Models

Generally, it's considered a best practice to keep your core business logic and validation in your models rather than moving it to forms when switching to custom views. Here's why:

1. Single Source of Truth: Models represent your data structure and business rules. Keeping validation there ensures consistency across all entry points.

2. DRY Principle: If validation is in your models, it applies everywhere the model is used - whether through forms, API, command line, or other interfaces.

3. Data Integrity: Model-level validation protects your database regardless of how data is being created or modified.

When to Use Form Validation

Form validation should typically handle:

• User interface concerns (formatting, display-specific rules)
• Input preprocessing before it reaches the model
• Validation that depends on the specific context of that form
• Field combinations that only exist in that particular form

Best Practice Approach

Consider a layered validation approach:

PYTHONCopied
# In your model
def clean(self):
    # Core business logic validation
    if self.start_date > self.end_date:
        raise ValidationError("End date must be after start date")
    
    # Other critical business rules...

PYTHONCopied
# In your form
def clean(self):
    cleaned_data = super().clean()
    
    # Form-specific validation
    if cleaned_data.get('special_option') and not cleaned_data.get('required_for_special'):
        self.add_error('required_for_special', 'This field is required when special option is selected')
    
    return cleaned_data

Since you've already implemented your business logic in full_clean(), you're on the right track. When creating your custom views and forms, you can leverage this existing validation rather than duplicating or moving it.