order.rb

class Order < ActiveRecord::Base
  belongs_to :person
  has_and_belongs_to_many :items
  validate :requires_at_least_one_item
  
  def requires_at_least_one_item
    errors.add(:items, "requires at least one Item") if items.empty?
  end
end

order_spec.rb

require 'spec_helper'

describe Order do
  it {should belong_to(:person)}
  it {should have_and_belong_to_many(:items)}
  
  it "should have at least 1 item" do
    o = Factory.build(:order, :items => [])    
    o.should_not be_valid
    o.errors[:items].should_not be_empty
  end
end