lib.rb

# Application namespace
class Manager
  def creating_community(*args, &blk)
    CreatingCommunity.new(*args).call(&blk)
  end

  def loading_community(*args, &blk)
    LoadingCommunity.new(*args).call(&blk)
  end

  def updating_community(*args, &blk)
    UpdatingCommunity.new(*args).call(&blk)
  end
end

# Models
class Manager::User < Struct.new(:id, :email)
end

class Manager::Community < Struct.new(:id, :name)
end

class Manager::CommunityManager < Struct.new(:id, :user, :community)
  def is?(user)
    self.user == user
  end
end

# Contexts
class Manager::CreatingCommunity
  SuccessResponse = Struct.new(:community_id)
  FailureResponse = Struct.new(:community, :errors)

  def initialize(user_id, attributes = {})
    @user_id = user_id
    @attributes = attributes
  end

  def call(&block)
    # NOTE: declare on top of context
    back = On.new(:success, :failure, &block)

    user      = repo.user.get(@user_id)

    community = repo.community.new(attributes)
    repo.community.save community

    manager   = repo.community_manager.new user, community
    repo.community_manager.save manager

    back.call :success, SuccessResponse.new(community.id)

  rescue SavingError => e
    back.call :failure, FailureResponse.new(community, e.errors)
  end
end

class Manager::LoadingCommunity
  # NOTE: POROs should be private
  SuccessResponse = Struct.new(:community, :community_manager)

  def initialize(community_id)
    @community_id = community_id
  end

  def call(&block)
    # NOTE: declare on top of context
    back = On.new(:success, :not_found, &block)

    # NOTE: could be a habit (nested context)
    community = repo.communitiy.get(@community_id)
    community_manager = repo.community_manager.find community

    if community
      back.call :success, SuccessResponse.new(community, community_manager)
    else
      back.call :not_found
    end
  end
end


class Manager::UpdatingCommunity
  SuccessResponse = Struct.new(:community_id)
  FailureResponse = Struct.new(:community, :errors)

  def initialize(user_id, community_id, attributes = {})
    @user_id      = user_id
    @community_id = community_id
    @attributes   = attributes
  end

  def call(&block)
    # NOTE: declare on top of context
    back = On.new(:success, :unauthorized, :failure, &block)

    user              = repo.user.get(@admin_id)

    # NOTE: could be a habit (nested context)
    community         = repo.communitiy.get(@community_id)
    community_manager = repo.community_manager.find(community)

    back.call(:not_found) && return if community.nil?

    if community_manager.is?(user)
      community.update_attributes @attributes

      repo.community.save community
      back.call :success, SuccessResponse.new(community.id)
    else
      back.call :unauthorized
    end

  rescue SavingError => e
    back.call :failure, FailureResponse.new(community, e.errors)
  end
end

rails.rb

# config/initializers/manager.rb
manager = Manager.new(magically_loaded_config_object)
Rails.configuration.manager = manager

class ApplicationController
  def manager
    Rails.configuration.manager
  end
end


# some controller
class CommunityController < ApplicationController

  def show
    manager.loading_community(params[:id]) do |callback|
      callback.on :success do |result|
        expose :community, CommunityPresenter.new(result.community, result.community_manager)
      end

      callback.on :not_found do
        raise ActionController::RoutingError
      end
    end
  end

  def new
    expose :community, CommunityFormPresenter.new
  end

  def create
    manager.creating_community(current_user.id, params[:community]) do |callback|
      callback.on :success do |result|
        redirect_to community_url(result.community_id)
      end

      callback.on :failure do |result|
        expose :community, CommunityFormPresenter.new(result.community, result.errors)
        render :new
      end
    end
  end

  def edit
    manager.loading_community(params[:id]) do |callback|
      callback.on :success do |result|
        expose :community, CommunityFormPresenter.new(result.community)
      end

      callback.on :not_found do
        raise ActionController::RoutingError
      end
    end
  end

  def update
    manager.updating_community(current_user.id, params[:id], params[:community]) do |callback|
      callback.on :success do |result|
        redirect_to edit_community_url(result.community_id)
      end

      callback.on :failure do |result|
        expose :community, CommunityFormPresenter.new(result.community, result.errors)
        render :edit
      end

      callback.on :not_found do
        raise ActionController::RoutingError
      end

      callback.on :unauthorized do
        render :status => rand
      end
    end
  end

end