user.rb

class User < ActiveRecord::Base

  acts_as_authentic

  attr_accessor :old_password

  validates_each :old_password, :on => :update, :if => :crypted_password_changed? do |record, attr, old_password|
    unless (old_password.nil? || record.valid_old_password?(old_password))
      record.errors.add attr, self.acts_as_authentic_config[:old_password_did_not_match_message]
    end
  end

  def valid_old_password?(old_password)
    params = [crypted_password_was, old_password, password_salt_was]
    self.class.acts_as_authentic_config[:crypto_provider].matches?(*params)
  end

end

user_spec.rb

require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')

describe User, "updating password" do
  
  before :each do
    @user = User.spawn
    @user.password = "111111"
    @user.password_confirmation = "111111"
    @user.save!
    @incurrect_set = {:password=>"123", :password_confirmation=>"123", :old_password=>"incorrect"}
    @correct_set = {:password=>"123", :password_confirmation=>"123", :old_password=>"111111"} 
  end

  it "should be invalid with empty old_password" do
    @user.update_attributes(:password=>"123", :password_confirmation=>"123", :old_password=>"").should == false
  end
  
  it "should be invalid with incorrent old_password" do
    @user.update_attributes(@incurrect_set).should == false
  end

  it "should update password without old_password" do
    @user.update_attributes(:password=>"123", :password_confirmation=>"123").should == true
    @user.valid_password?("123").should == true
  end
  
  it "should update password with correct old_password and valid new password" do
    @user.update_attributes(@correct_set).should == true
    @user.valid_password?("123").should == true
  end
  
  it "should add configured error message if old_password didn't match" do
    User.acts_as_authentic_config[:old_password_did_not_match_message] = "custome message"
    @user.update_attributes(:password=>"123", :password_confirmation=>"123", :old_password=>"incorrect")
    @user.errors.on(:old_password).should == "custome message"
  end

  it "should work several times with incorrect old_password" do
    @user.update_attributes(@incurrect_set).should == false
    @user.update_attributes(@incurrect_set).should == false
  end
  
  it "should work several times with correct old_password" do
    @user.update_attributes(@correct_set).should == true
    @correct_set[:old_password] = "123"
    @user.update_attributes(@correct_set).should == true
  end
end

describe User, "updating non password fields" do
  before :each do
    @user = User.spawn
    @user.password = "111111"
    @user.password_confirmation = "111111"
    @user.save!
  end

  it "should update without old_password" do
    @user.update_attributes(:last_name=>'saldon').should == true
  end
  
  it "should update with empty old_password" do
    @user.update_attributes(:last_name=>'saldon', :old_password=>"").should == true
  end
  
  it "should update with incorrect old_password" do
    @user.update_attributes(:last_name=>'saldon', :old_password=>"231").should == true
  end
  
  it "should update with correct old_password" do
    @user.update_attributes(:last_name=>'saldon', :old_password=>"111111").should == true
  end
end