ruby script/plugin install http://validation-scenarios.googlecode.com/svn/tags/validation-scenarios/

===Usage

First, in your models, you declare validation scenarios details using when_scenario_is method. In the following example, both User and Address models have user_registration and profile_update scenarios declared:

  class User < ActiveRecord::Base
    has_one :address
    
    when_scenario_is :user_registration do |this|
      this.validates_presence_of :email
      this.validates_associated :address
    end

    when_scenario_is :profile_update do |this|
      this.validates_presence_of :first_name :unless => :something_here
    end 
  end

  class Address < ActiveRecord::Base 
    belongs_to :user
    
    when_scenario_is :user_registration do |this|
      this.validates_numericality_of :postcode
    end
   
    when_scenario_is :profile_update do |this|
      this.validates_presence_of :street_address_1
      this.validates_presence_of :city
    end
  end

Then, when you want to process, for example, the user_registration scenario - you just start it using: ActiveRecord::Base.with_validation_scenario method. In the following example it validates User object per rules declared for user_registration scenario.

Please also note that associated Address instance is also aware that the current validation scenario is user_registration. That's why the following test passes:

 ActiveRecord::Base.with_validation_scenario(:user_registration) do
    user = User.new(:email => "foo@bar.baz")
    user.build_address(:postcode => 'intentionally_not_numerical')
    assert !user.save
    assert user.errors.invalid?(:address)
    assert_match(/is not a number/i, user.address.errors.on(:postcode))
  end

There is currently no advanced documentation for this plugin.