ModelForm Meta Class Validation

Based on the Django documentation for ModelForms, I can provide some context on the validation process and Meta class usage.

Current Behavior Difference

You've correctly identified a difference in behavior between Model and ModelForm classes:

• Model classes validate attributes in their Meta class and raise TypeError for invalid attributes
• ModelForm classes silently ignore invalid attributes in their Meta class

ModelForm Validation Process

The ModelForm validation process consists of two main steps:

1. Validating the form itself
2. Validating the model instance

This validation is triggered when calling is_valid() or accessing the errors attribute, and the model validation is triggered right after the form's clean() method is called.

Considerations for Your Proposal

While making ModelForm's Meta class validation consistent with Model's behavior seems logical, there are some important considerations:

1. Backward Compatibility: As noted in your ticket, this change could break existing applications that rely on custom attributes in the Meta class.

2. Current Usage Patterns: The ModelForm Meta class is commonly used for configuration options like:

   • Specifying the model
   • Selecting fields to include/exclude
   • Overriding default fields
   • Specifying widgets
   • Defining error messages

3. Error Message Precedence: The documentation specifically mentions that error messages defined at the form field level or form Meta level take precedence over those defined at the model field level.

Potential Solution

A registration mechanism similar to what was proposed in ticket #5793 could be a good compromise:

• It would allow for validation of attributes
• It would provide a way to register custom attributes
• It would maintain backward compatibility if implemented carefully

This approach would make the behavior more consistent between Model and ModelForm while still allowing for flexibility.

For example, the current way to override error messages in the Meta class is documented:

PYTHONCopied
from django.core.exceptions import NON_FIELD_ERRORS
from django.forms import ModelForm

class ArticleForm(ModelForm):
    class Meta:
        error_messages = {
            NON_FIELD_ERRORS: {
                "unique_together": "%(model_name)s's %(field_labels)s are not unique.",
            }
        }

A similar pattern could be used for registering custom attributes.