## size.rb

  after_create :add_size_to_products_in_category


  protected
  def add_size_to_products_in_category
    self.category.products.each do |product|
      product.sizes << self
    end
  end


## size_spec.rb
  let(:category) { FactoryGirl.build :category }
  let(:size) { FactoryGirl.build :size }
  let(:product) { FactoryGirl.build :product }

  it "should add size to all products" do
    product.save.should be_true
    sizes = product.sizes.count
    size = Size.new :name => "Another size", :category => category
    size.save.should be_true
    product.sizes.count.should eql(sizes + 1) # fails here saying it gets 0 expecting 1
  end