order.rb

class Order < ActiveRecord::Base
  has_many :line_items, :dependent => :destroy, :order => "position"
  
  accepts_nested_attributes_for :line_items, :allow_destroy => true, :reject_if => lambda { |attributes| attributes["amount"].blank? }
  
  validates_associated :line_items

  private
  
  def validate
    self.errors.add_to_base("An order needs at least one line item") if self.line_items.empty?
  end
end

order_spec.rb

describe Order, "being valid" do
  
  it "requires at least one line item" do
    order = Order.new
    order.should_not be_valid
    order.errors.on_base.should include("An order needs at least one line item")
  end
  
end