contact.rb

class Contact < ActiveRecord::Base

  validates_presence_of :start_at, :message=>"<strong>Data de Ida</strong> deve ser preenchida corretamente."
  validates_presence_of :finish_at, :message=>"<strong>Data de Volta</strong> deve ser preenchida corretamente."
  
  validate :check_dates

  def start_at=(start_at)
    super(format_date(start_at))
  end
  
  def finish_at=(finish_at)
    super(format_date(finish_at))
  end
  
  def start_at
      super.strftime("%d/%m/%Y")
    rescue
      nil
  end
  
  def finish_at
      super.strftime("%d/%m/%Y")
    rescue
      nil
  end
  
  protected
  def format_date(date)
    if date.class.eql? String
      if date =~ /^[0-9]+\/[0-9]+\/[0-9]{4}$/
        time = date.split('/')
        [time[1],time[0],time[2]].join('/')
      else
        nil
      end
    else
      date
    end
  end
  
  def check_dates
    if !finish_at.nil? and !start_at.nil? and finish_at < start_at
      @errors.add(:start_at, "<strong>Data de Volta</strong> não pode ser anterior à <strong>Data de Ida</strong>.")
    end
  end
end

contact_test.rb

require File.dirname(__FILE__) + '/../test_helper'

class ContactTest < Test::Unit::TestCase
  def test_getting_start_at
    c = Contact.new
    c.start_at = "10/02/2008"
    assert_equal(c.start_at, "10/02/2008")
  end
  
  def test_getting_finish_at
    c = Contact.new
    c.finish_at = "10/02/2008"
    assert_equal(c.finish_at, "10/02/2008")
  end
  
  def test_validating_dates
    c = Contact.new
    c.start_at = "10/02/2008"
    c.finish_at = "09/02/2008"
    
    assert(!c.valid?)
    assert_equal("<strong>Data de Volta</strong> não pode ser anterior à <strong>Data de Ida</strong>.", c.errors.on(:start_at))
  end
  
  def test_validating_format_of_dates
    c = Contact.new
    c.start_at = "a/02/2008"
    assert_equal(nil, c.start_at)
    
    c.finish_at = "a/02/2008"
    assert_equal(nil, c.finish_at)
    
    assert(!c.valid?)
    
    assert_equal("<strong>Data de Ida</strong> deve ser preenchida corretamente.", c.errors.on(:start_at))
    assert_equal("<strong>Data de Volta</strong> deve ser preenchida corretamente.", c.errors.on(:finish_at))
  end
end