Controller refactoring (my presentation at a local Ruby meetup)

01-classic.rb

#################
# CLASSIC STYLE #
#################

class SessionsController < ApplicationController
  # ...

  def create
    if user = AuthenticateUser.call(params[:username], params[:password])
      sign_in!(user)
      redirect_to root_path
    else
      flash[:error] = "Invalid username or password"
      render :new
    end
  end

  # ...
end

class PostsController < ApplicationController
  # ...

  def create
    @post = CreatePost.call(params[:post])
    if @post.valid?
      redirect_to @post
    else
      render :new
    end
  end

  def update
    @post = UpdatePost.call(params[:id], params[:post])
    if @post.valid?
      redirect_to @post
    else
      render :edit
    end
  end

  # ...
end


# Downsides:
# - Ugly `if` conditionals in all CRUD actions
# - Not very DRY

02-responders.rb

##############
# RESPONDERS #
##############

class SessionsController < ApplicationController
  # ...

  def create
    if user = AuthenticateUser.call(params[:username], params[:password])
      sign_in!(user)
      redirect_to root_path
    else
      flash[:error] = "Invalid username or password"
      render :new
    end
  end

  # ...
end

class PostsController < ApplicationController
  # ...

  def create
    respond_with CreatePost.call(params[:post])
  end

  def update
    respond_with UpdatePost.call(params[:id], params[:post])
  end

  # ...
end


# Upsides:
# - No `if` conditionals 🎉
# - We're pretty DRY now
#
# Downsides:
# - We cannot use it for e.g. authentication, because those validatons don't happen on the model
# - We still have to repeat `respond_with` (but not a big deal)
#
# This is the best we can do for classic full-stack controller

03-json-api.rb

############
# JSON API #
############

class SessionsController < ApplicationController
  # ...

  def create
    if user = AuthenticateUser.call(params[:username], params[:password])
      render json: user
    else
      render json: {errors: ["Invalid username or password"]}, status: 400
    end
  end

  # ...
end

class PostsController < ApplicationController
  # ...

  def create
    post = CreatePost.call(params[:post])
    if post.valid?
      render json: post
    else
      render json: {errors: post.errors}, status: 400
    end
  end

  def update
    post = UpdatePost.call(params[:id], params[:post])
    if post.valid?
      render json: post
    else
      render json: {errors: post.errors}, status: 400
    end
  end

  # ...
end

# Again not DRY, but it's visible how we can DRY it up

04-reflection.md

Q: What are service objects? A: They are simple Ruby classes.

Q: Why do validation errors appear? A: Because the record was invalid.

Q: Why was the record invalid? A: Because the params were invalid.

Q: Who accepts the params A: Service objects.

Q: What do Ruby classes do when you give them invalid arguments? A: They raise an ArgumentError.

Validation errors are exceptional by nature, so it makes sense to raise errors

05-errors.rb

##########
# ERRORS #
##########

class SessionsController < ApplicationController
  # ...

  def create
    render json: AuthenticateUser.call(params[:username], params[:password])
  end

  # ...
end

class PostsController < ApplicationController
  # ...

  def create
    render json: CreatePost.call(params[:post])
  end

  def update
    render json: UpdatePost.call(params[:id], params[:post])
  end

  # ...
end

class ApplicationController < ActionController::Base
  # MyApp::Error::Validation < MyApp::Error
  # MyApp::Error::Unauthorized < MyApp::Error
  rescue_from MyApp::Error do |error|
    render json: {errors: error.messages}, status: error.status
  end
end

# Service classes raise validation or unauthorized errors.
# This really looks neat now. There is still the `render :json` duplication,
# but that's the best we can do in Rails.
#
# However, we can do even better if we switch frameworks.

06-roda.rb

########
# Roda #
########

class MyApp < Roda
  plugin :json, serializer: Serializer, classes: Serializer::CLASSES
  plugin :error_handler

  route do |r|
    # ...

    r.get "/login" do
      AuthenticateUser.call(params[:username], params[:password])
    end

    # ...

    r.post "/posts" do
      CreatePost.call(params[:post])
    end

    r.patch "/posts/:id" do |id|
      UpdatePost.call(id, params[:post])
    end

    # ...
  end

  error do |error|
    if MyApp::Error === error
      response.status = error.status
      error
    else
      raise error
    end
  end
end


class Serializer
  CLASSES = [Hash, Array, ActiveRecord::Base, ActiveRecord::Relation, MyApp::Error]

  def self.call(object)
    case object
    when Hash, Array
      object.to_json
    else
      yaks.call(object)
    end
  end
end