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 as_json(options = {})
    {: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

credentials_controller.rb

class CredentialsController < ApplicationController
  def create
    credential = Credential.new(params)
    if credential.save
      render :json => credential
    else
      render :json => {:error => errors_for(credential)}, :status => :unprocessable_entity
    end
  end

  private

  def errors_for(object)
    object.errors.map {|k, m| "#{k} #{m}" }
  end
end

I really love this technique.

One additional thing you can do is move the errors_for and as_json logic to separate object - CredentialsSerializer:

https://gist.github.com/2016981/289e50c4c20996486015e02b6f0b95059d0db3c0