Sample code from my Using OO to Manage Control Flow post

credential.rb

class Credential
  include ActiveModel::Validations

  attr_accessor :screen_name, :oauth_token, :oauth_secret, :token, :description

  validates_presence_of :screen_name, :oauth_token, :oauth_secret, :message => 'required'
  validate :user_exists, :unless => :errors?

  def initialize(attributes = {})
    attributes.each { |k, v| set_recognized_attribute(k, v) }
  end

  def save
    valid? && create_device
  end

  def api_key
    @device.api_key
  end

  private

  def errors?
    errors.any?
  end

  def set_recognized_attribute(name, value)
    setter_method = "#{name}="
    self.send(setter_method, value) if respond_to?(setter_method)
  end

  def user
    @user ||= User.by_screen_name(screen_name).where({
      :oauth_token  => oauth_token,
      :oauth_secret => oauth_secret
    }).first
  end

  def create_device
    @device = Device.find_or_create_by_token!({
      :token       => token,
      :description => description,
      :user_id     => user.id
    })
    !@device.nil?
  end

  def user_exists
    errors.add(:user, 'not found') unless user.present?
  end
end

credential_serializer.rb

class CredentialSerializer
  def initialize(credential)
    @credential = credential
  end

  def to_json
    {:api_key => @credential.api_key}.to_json
  end

  def errors
    @credential.errors.map { |k, m| "#{k} #{m}" }
  end
end

credentials_controller.rb

class CredentialsController < ApplicationController
  def create
    credential = Credential.new(params)
    serializer = CredentialSerializer.new(credential)

    respond_with serializer
  end
end