class Post < ActiveRecord::Base
  validates_presence_of :title
  validates_presence_of :publishing_date, :if => :published
end

class PostTest < ActiveSupport::TestCase
  
  context "Post that is NOT going to be published" do
    should validate_presence_of(:title)
    should_not validate_presence_of(:publishing_date)
  end
  
  context "Post that is going to be published" do
    subject do
      Post.new(:title => 'Valid Title', :published => true)
    end
    should validate_presence_of(:publishing_date)
  end
      
end

Shoulda with TestUnit: Conditional Validation using Shoulda/Subject

validation

Rails