Modern Rails 8 Architecture Patterns

Architecture Decision Tree

Where should this code go?
|
+- View/display formatting?       -> Presenter (@presenter-agent)
+- Complex business logic?        -> Service Object (@service-agent)
+- Complex database query?        -> Query Object (@query-agent)
+- Shared behavior across models? -> Concern (/rails-concern skill)
+- Authorization logic?           -> Policy (@policy-agent)
+- Reusable UI with logic?        -> ViewComponent (@viewcomponent-agent)
+- Async/background work?         -> Job (@job-agent, /solid-queue-setup skill)
+- Complex form (multi-model)?    -> Form Object (@form-agent)
+- Transactional email?           -> Mailer (@mailer-agent)
+- Real-time/WebSocket?           -> Channel (/action-cable-patterns skill)
+- Data validation only?          -> Model (@model-agent)
+- HTTP request/response only?    -> Controller (@controller-agent)

Layer Responsibilities

Layer	Responsibility	Should NOT contain
Controller	HTTP, params, response	Business logic, queries
Model	Data, validations, relations	Display logic, HTTP
Service	Business logic, orchestration	HTTP, display logic
Query	Complex database queries	Business logic
Presenter	View formatting, badges	Business logic, queries
Policy	Authorization rules	Business logic
Component	Reusable UI encapsulation	Business logic
Job	Async processing	HTTP, display logic
Form	Complex form handling	Persistence logic
Mailer	Email composition	Business logic
Channel	WebSocket communication	Business logic

When NOT to Abstract

Situation	Keep It Simple	Don't Create
Simple CRUD (< 10 lines)	Keep in controller	Service object
Used only once	Inline the code	Abstraction
Simple query with 1-2 conditions	Model scope	Query object
Basic text formatting	Helper method	Presenter
Single model form	form_with model:	Form object
Simple partial without logic	Partial	ViewComponent

When TO Abstract

Signal	Action
Same code in 3+ places	Extract to concern/service
Controller action > 15 lines	Extract to service
Model > 300 lines	Extract concerns
Complex conditionals	Extract to policy/service
Query joins 3+ tables	Extract to query object
Form spans multiple models	Extract to form object

See /extraction-timing skill for detailed extraction guidance.

Core Patterns

Skinny Controllers

# GOOD: Thin controller delegates to service
class OrdersController < ApplicationController
  def create
    result = Orders::CreateService.call(user: current_user, params: order_params)
    if result.success?
      redirect_to result.data, notice: t(".success")
    else
      flash.now[:alert] = result.error
      render :new, status: :unprocessable_entity
    end
  end
end

Result Objects for Services

All services return a consistent Result object:

Result = Data.define(:success, :data, :error) do
  def success? = success
  def failure? = !success
end

Multi-Tenancy by Default

# GOOD: Scoped through account
def index
  @events = current_account.events.recent
end

Rails 8 Specific Features

Feature	Purpose	Skill/Agent
Authentication	has_secure_password generator	/authentication-flow
Background Jobs	Solid Queue (database-backed)	/solid-queue-setup, @job-agent
Real-time	Action Cable + Solid Cable	/action-cable-patterns
Caching	Solid Cache (database-backed)	/caching-strategies
Assets	Propshaft + Import Maps	(built-in)
Deployment	Kamal 2 + Thruster	(built-in)

Testing Strategy by Layer

Layer	Test Type	Focus
Model	Unit	Validations, scopes, methods
Service	Unit	Business logic, edge cases
Query	Unit	Query results, tenant isolation
Presenter	Unit	Formatting, HTML output
Controller	Request	Integration, HTTP flow
Component	Component	Rendering, variants
Policy	Unit	Authorization rules
System	E2E	Critical user paths

New Feature Checklist

1. Model - Define data structure (@migration-agent, @model-agent)
2. Policy - Add authorization rules (@policy-agent)
3. Service - Create for complex logic (@service-agent)
4. Query - Add for complex queries (@query-agent)
5. Controller - Keep it thin (@controller-agent)
6. Presenter - Format for display (@presenter-agent)
7. Component - Build reusable UI (@viewcomponent-agent)
8. Mailer - Add transactional emails (@mailer-agent)
9. Job - Add background processing (@job-agent)

References

• See layer-interactions.md for layer communication patterns
• See service-patterns.md for service object patterns
• See query-patterns.md for query object patterns
• See error-handling.md for error handling strategies
• See testing-strategy.md for comprehensive testing