Service Patterns

Rails Service Patterns

Extract business logic that spans multiple models or involves external systems into service objects to promote single responsibility, testability, and reusability.

When to Extract a Service

Indicators for service extraction:

• Business logic spans multiple models
• An operation involves external API calls
• Multi-step operations need transaction safety
• Logic violates single responsibility in a model
• Controller actions grow beyond simple CRUD
• The same logic is needed in multiple places (controller, job, rake task)

Keep logic in the model when:

• It naturally fits as a model concern (validations, scopes, associations)
• A single ActiveRecord operation is sufficient
• A model callback is more appropriate

Pattern Overview

Callable Concern

DRY up the repeated .call class method across services with a shared concern. See examples/callable.rb for the full implementation.

module Callable
  extend ActiveSupport::Concern

  class_methods do
    def call(...)
      new(...).call
    end
  end
end

Include in every service to get MyService.call(args) delegation to new(args).call.

Basic Service Object

A minimal service with the Callable concern and Result return type:

class CreatePost
  include Callable

  def initialize(params, user)
    @params = params
    @user = user
  end

  def call
    post = @user.posts.build(@params)

    if post.save
      NotifyFollowersJob.perform_later(post.id)
      Result.success(post: post)
    else
      Result.failure(errors: post.errors.full_messages)
    end
  end
end

See examples/create_post.rb for the full example.

File Organization

app/
├── services/
│   ├── concerns/
│   │   └── callable.rb
│   ├── result.rb
│   ├── create_post.rb
│   ├── transfer_funds.rb
│   └── github/
│       └── create_issue.rb
├── queries/
│   └── active_users_query.rb
└── forms/
    └── registration_form.rb

Testing Services

Test services through their .call interface. Assert on Result success/failure and side effects. Both RSpec and Minitest examples are available in examples/:

• examples/create_post_spec.rb — RSpec example
• examples/create_post_test.rb — Minitest example

Design Principles

1. Single responsibility — One service does one thing
2. Clear interface — .call class method with explicit parameters
3. Consistent return type — Always return a Result object
4. Proper error handling — Rescue specific exceptions, never swallow errors
5. Testability — Easy to test in isolation, injectable dependencies
6. Immutability — Never modify input arguments

Additional Resources

Reference Files

For detailed implementations, patterns, and alternatives:

• references/result-object.md — Full Result implementation, Struct-based alternative, and dry-monads approach
• references/transaction-services.md — Transaction wrapping, nested transactions, and side-effect management
• references/external-api-services.md — API wrapper pattern with error handling, timeouts, and retry guidance
• references/query-objects.md — Query object pattern, composability, and when to prefer scopes
• references/form-objects.md — Multi-model forms with ActiveModel, validations, and form builder integration

Example Files

Working code in examples/:

• examples/callable.rb — Callable concern to DRY up .call boilerplate
• examples/create_post.rb — Basic service object
• examples/transfer_funds.rb — Transaction service
• examples/registration_form.rb — Form object
• examples/create_post_spec.rb — RSpec test example
• examples/create_post_test.rb — Minitest test example

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