advanced-rails-patterns

Thin controllers delegate business logic to service objects and models, keeping action methods focused on HTTP concerns. Move validation, calculations, and state changes into service objects or model methods, then call them from controllers with strong parameters. This pattern preserves test coverage while enforcing Rails conventions and Sandi Metz rules for simpler, more testable code.

Nested attributes let parent models accept and persist child record data in a single request. Configure `accepts_nested_attributes_for` on the parent model, define strong parameters for nested data, and structure form fields to match the nested param keys. This pattern handles complex multi-level form submissions like orders with line items while maintaining validation and test coverage.

Polymorphic associations let a model belong to multiple unrelated parent types using a single foreign key. Add `polymorphic: true` to the belongs_to declaration and corresponding columns to the table, then use `belongs_to :association, polymorphic: true` with type and id columns. This enables one-of-type handling for flexible model relationships without duplicating code.

Use service objects for multi-step operations spanning several models, external dependencies, or complex business logic that doesn't belong to a single entity. Service objects keep models focused on data representation and relationships while encapsulating orchestration logic. This separation makes refactoring easier, tests faster, and code alignment with Sandi Metz rules clearer.

Yes. These patterns preserve existing test coverage during refactoring by extracting logic incrementally and running tests at each step. Refactor controllers, models, and service objects one piece at a time, keeping tests green to catch regressions. The approach ensures functionality remains intact while gradually moving code toward thin controllers and idiomatic Ruby conventions.

Concerns are modules that extract shared behavior from models into reusable mixins. Define a concern in `app/models/concerns/`, include it in models with `include ConcernName`, and move related methods and validations into it. Concerns reduce duplication across models and improve organization without changing the refactoring outcome or test coverage.