rails-model-design

Validations and associations form the foundation of ActiveRecord models. Use belongs_to, has_many, and custom validators to enforce data integrity at the model layer, then add scopes and domain methods to encapsulate business logic and keep controllers thin.

Scopes and class methods encapsulate reusable query logic in the model. Define scopes for common filters like status or date ranges, and use class methods for complex queries. This keeps query logic in one place and makes controllers easier to read.

Domain methods are instance methods that express business behavior—like confirm! or calculate_total—directly on the model. They keep business logic in the model layer rather than scattered across controllers or services, making code more maintainable.

Callbacks automate actions when records change, but overuse creates hidden dependencies. Use them sparingly for simple triggers; for complex workflows, prefer explicit domain methods or service objects to keep model behavior transparent.

A fat model approach works well for medium to large Rails applications when combined with clear separation of concerns. Group related validations, scopes, and domain methods logically; extract unrelated business logic into service objects or concerns when a model grows beyond 200-300 lines.

Test validations with valid and invalid attribute combinations, scopes with filtered datasets, and associations by checking related records are correctly loaded. Use RSpec or Minitest to verify each layer independently, ensuring models behave correctly before controllers use them.