django-forms-validation-tdd

Testing & Quality

Test-drive Django validation across model constraints, forms, ModelForms, views, template errors, duplicate handling, and redirects. Use when adding or refactoring Django forms, surfacing validation errors, deciding model-vs-form-vs-view responsibility, or preventing invalid input from being saved.

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Django Forms Validation TDD

Use this skill to place validation tests at the layer that owns the rule, then prove the user-facing error path still works. Django validation often crosses model, form, view, and template boundaries; keep those responsibilities explicit.

Source Traceability

Primary source: Harry Percival, Test-Driven Development with Python, 3rd ed. Guidance is transformed and paraphrased from chapters 14-16 and Appendix A, especially model constraints, form validation, ModelForm tradeoffs, view-level error surfacing, and duplicate item handling.

Workflow

Classify the validation rule.
- Data integrity belongs near the model or database constraint.
- User input shape and presentation belong in a form.
- Request branching and redirects belong in the view.
- Error rendering belongs in the template.
Write the lowest useful failing test.
- Model constraint or validation test for persistence rules.
- Form test for field errors, cleaning, and save behavior.
- View test for HTTP status, redirect, template, and invalid-save prevention.
- Functional test only for a critical user journey.
Prove invalid data is not persisted.
- Check both the error response and database state when the risk is saving bad input.
- For uniqueness, cover duplicate behavior at the level that actually enforces it.
Refactor toward the right boundary.
- Move validation from views to forms when the view is parsing form details.
- Move integrity rules to constraints when the database must protect them.
- Keep presentation logic in templates unless Python code clearly owns it.

Read validation-patterns.md for layer choices and common Django edge cases.

Decision Rules

If the rule must hold outside HTTP, test it below the view.
If save() can bypass validation, test the path that actually runs validation or enforce the rule with constraints.
If a ModelForm couples the UI to unwanted model details, use a plain form or customize fields deliberately.
If a refactor changes field names, IDs, or templates, keep one user-facing check for the rendered contract.
If duplicate handling depends on the database, include the database-level behavior in coverage.

Guardrails

Do not rely only on Selenium for validation cases.
Do not test every HTML string when form errors can be inspected directly.
Do not assume Django model validation runs automatically on every save.
Do not swallow IntegrityError without a user-facing path and a test.

Verification

Before finishing, record:

Rule ownership by layer.
Tests added or updated at model/form/view/browser level.
Invalid-save prevention evidence where relevant.
Focused Django test command and result.