Where should this code go?
│
├─ Is it view/display formatting?
│   └─ → Presenter (see: rails-presenter skill)
│
├─ Is it complex business logic?
│   └─ → Service Object (see: rails-service-object skill)
│
├─ Is it a complex database query?
│   └─ → Query Object (see: rails-query-object skill)
│
├─ Is it shared behavior across models?
│   └─ → Concern (see: rails-concern skill)
│
├─ Is it authorization logic?
│   └─ → Policy (see: authorization-pundit skill)
│
├─ Is it reusable UI with logic?
│   └─ → ViewComponent (see: viewcomponent-patterns skill)
│
├─ Is it async/background work?
│   └─ → Job (see: solid-queue-setup skill)
│
├─ Is it a complex form (multi-model, wizard)?
│   └─ → Form Object (see: form-object-patterns skill)
│
├─ Is it a transactional email?
│   └─ → Mailer (see: action-mailer-patterns skill)
│
├─ Is it real-time/WebSocket communication?
│   └─ → Channel (see: action-cable-patterns skill)
│
├─ Is it data validation only?
│   └─ → Model (see: rails-model-generator skill)
│
└─ Is it HTTP request/response handling only?
    └─ → Controller (see: rails-controller skill)

Layer Interaction Flow

┌─────────────────────────────────────────────────────────────┐
│                        REQUEST                               │
└─────────────────────────┬───────────────────────────────────┘
                          ▼
┌─────────────────────────────────────────────────────────────┐
│                     CONTROLLER                               │
│  • Authenticate (Authentication concern)                     │
│  • Authorize (Policy)                                        │
│  • Parse params                                              │
│  • Delegate to Service/Query                                 │
└──────────┬─────────────────────────────────┬────────────────┘
           │                                 │
           ▼                                 ▼
┌─────────────────────┐           ┌─────────────────────┐
│      SERVICE        │           │       QUERY         │
│  • Business logic   │           │  • Complex queries  │
│  • Orchestration    │           │  • Aggregations     │
│  • Transactions     │           │  • Reports          │
└──────────┬──────────┘           └──────────┬──────────┘
           │                                 │
           ▼                                 ▼
┌─────────────────────────────────────────────────────────────┐
│                        MODEL                                 │
│  • Validations  • Associations  • Scopes  • Callbacks       │
└─────────────────────────┬───────────────────────────────────┘
                          │
           ┌──────────────┴──────────────┐
           ▼                             ▼
┌─────────────────────┐       ┌─────────────────────┐
│     PRESENTER       │       │    VIEW COMPONENT   │
│  • Formatting       │       │  • Reusable UI      │
│  • Display logic    │       │  • Encapsulated     │
└──────────┬──────────┘       └──────────┬──────────┘
           │                             │
           └──────────────┬──────────────┘
                          ▼
┌─────────────────────────────────────────────────────────────┐
│                       RESPONSE                               │
└─────────────────────────────────────────────────────────────┘

ASYNC FLOWS:
┌─────────────────────┐       ┌─────────────────────┐
│        JOB          │       │      CHANNEL        │
│  • Background work  │       │  • Real-time        │
│  • Solid Queue      │       │  • WebSockets       │
└─────────────────────┘       └─────────────────────┘

EMAIL FLOWS:
┌─────────────────────┐
│       MAILER        │
│  • Transactional    │
│  • Notifications    │
└─────────────────────┘

See layer-interactions.md for detailed examples.

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

Project Directory Structure

app/
├── channels/            # Action Cable channels
├── components/          # ViewComponents (UI + logic)
├── controllers/
│   └── concerns/        # Shared controller behavior
├── forms/               # Form objects
├── helpers/             # Simple view helpers (avoid)
├── jobs/                # Background jobs (Solid Queue)
├── mailers/             # Action Mailer classes
├── models/
│   └── concerns/        # Shared model behavior
├── policies/            # Pundit authorization
├── presenters/          # View formatting
├── queries/             # Complex queries
├── services/            # Business logic
│   └── result.rb        # Shared Result class
└── views/
    └── layouts/
        └── mailer.html.erb  # Email layout

Core Principles

1. Skinny Controllers

Controllers should only:

Authenticate/authorize
Parse params
Call service/query
Render response

# GOOD: Thin controller
class OrdersController < ApplicationController
  def create
    result = Orders::CreateService.new.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

# BAD: Fat controller with business logic
class OrdersController < ApplicationController
  def create
    @order = Order.new(order_params)
    @order.user = current_user

    if @order.valid?
      inventory_available = @order.items.all? do |item|
        Product.find(item.product_id).inventory >= item.quantity
      end

      if inventory_available
        # ... more logic
      end
    end
  end
end

2. Rich Models, Smart Services

Models handle:

Validations
Associations
Scopes
Simple derived attributes

Services handle:

Multi-model operations
External API calls
Complex business rules
Transactions across models

3. Result Objects for Services

All services return a consistent Result object:

class Result
  attr_reader :data, :error, :code

  def initialize(success:, data: nil, error: nil, code: nil)
    @success = success
    @data = data
    @error = error
    @code = code
  end

  def success? = @success
  def failure? = !@success
end

4. Multi-Tenancy by Default

All queries scoped through account:

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

# BAD: Unscoped query
def index
  @events = Event.where(user_id: current_user.id)
end

When NOT to Abstract (Avoid Over-Engineering)

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

Signs of Over-Engineering

# OVER-ENGINEERED: Service for simple save
class Users::UpdateEmailService
  def call(user, email)
    user.update(email: email)  # Just do this in controller!
  end
end

# KEEP IT SIMPLE
class UsersController < ApplicationController
  def update
    if @user.update(user_params)
      redirect_to @user
    else
      render :edit
    end
  end
end

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

Pattern Selection Guide

Use Service Objects When:

Logic spans multiple models
External API calls needed
Complex business rules
Need consistent error handling
Logic reused across controllers/jobs

→ See rails-service-object skill for details.

Use Query Objects When:

Complex SQL/ActiveRecord queries
Aggregations and statistics
Dashboard data
Reports

→ See rails-query-object skill for details.

Use Presenters When:

Formatting data for display
Status badges with colors
Currency/date formatting
Conditional display logic

→ See rails-presenter skill for details.

Use Concerns When:

Shared validations across models
Common scopes (e.g., Searchable)
Shared callbacks (e.g., HasUuid)
Keep it single-purpose!

→ See rails-concern skill for details.

Use ViewComponents When:

Reusable UI with logic
Complex partials
Need testable views
Cards, tables, badges

→ See viewcomponent-patterns skill for details.

Use Form Objects When:

Multi-model forms
Wizard/multi-step forms
Search/filter forms
Contact forms (no persistence)

→ See form-object-patterns skill for details.

Use Policies When:

Resource authorization
Role-based access
Action permissions
Scoped collections

→ See authorization-pundit skill for details.

Rails 8 Specific Features

Authentication (Built-in Generator)

bin/rails generate authentication

Uses has_secure_password with Session model, Current class, and password reset flow.

→ See authentication-flow skill for details.

Background Jobs (Solid Queue)

Database-backed job processing, no Redis required.

→ See solid-queue-setup skill for details.

Real-time (Action Cable + Solid Cable)

WebSocket support with database-backed adapter.

→ See action-cable-patterns skill for details.

Caching (Solid Cache)

Database-backed caching, no Redis required.

→ See caching-strategies skill for details.

Other Rails 8 Defaults

Feature	Purpose
Propshaft	Asset pipeline (replaces Sprockets)
Importmap	JavaScript without bundling
Kamal	Docker deployment
Thruster	HTTP/2 proxy with caching

Anti-Patterns to Avoid

Anti-Pattern	Problem	Solution
God Model	Model > 500 lines	Extract services/concerns
Fat Controller	Logic in controllers	Move to services
Callback Hell	Complex model callbacks	Use services
Helper Soup	Massive helper modules	Use presenters/components
N+1 Queries	Unoptimized queries	Use `.includes()`, query objects
Stringly Typed	Magic strings everywhere	Use constants, enums
Premature Abstraction	Service for 3 lines	Keep in controller

→ See performance-optimization skill for N+1 detection.

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
Form	Unit	Validations, persistence
System	E2E	Critical user paths

→ See tdd-cycle skill for TDD workflow.

Quick Reference

New Feature Checklist

Model - Define data structure
Policy - Add authorization rules
Service - Create for complex logic (if needed)
Query - Add for complex queries (if needed)
Controller - Keep it thin!
Form - Use for multi-model forms (if needed)
Presenter - Format for display
Component - Build reusable UI
Mailer - Add transactional emails (if needed)
Job - Add background processing (if needed)

Refactoring Signals

Signal	Action
Model > 300 lines	Extract concern or service
Controller action > 15 lines	Extract service
View logic in helpers	Use presenter
Repeated query patterns	Extract query object
Complex partial with logic	Use ViewComponent
Form with multiple models	Use form object
Same code in 3+ places	Extract to shared module

Related Skills

Category	Skills
Data Layer	rails-model-generator, rails-query-object, database-migrations
Business Logic	rails-service-object, rails-concern, form-object-patterns
Presentation	rails-presenter, viewcomponent-patterns
Controllers	rails-controller, api-versioning
Auth	authentication-flow, authorization-pundit
Background	solid-queue-setup, action-mailer-patterns
Real-time	action-cable-patterns, hotwire-patterns
Performance	caching-strategies, performance-optimization
I18n	i18n-patterns
Testing	tdd-cycle

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

thibautbaissac-rails-architecture

Modern Rails 8 Architecture Patterns

Overview

Architecture Decision Tree