item.rb

class Item < ActiveRecord::Base
  has_many :orders, :through => :line_items
  validates_presence_of :name, :price
  before_save :round_price
  
  def round_price
    self.price = price.round(2)
  end
end

item_spec.rb

require 'spec_helper'

describe Item do
  it {should have_many(:orders).through(:line_items)}
  
  [:name, :price].each do |attr|
    it "must have a(n) #{attr}" do
      i = Factory.build(:item, attr.to_sym => nil)
      i.should_not be_valid
      i.errors[attr].should_not be_nil
    end
  end
  
  it "should store 2 decimals on price" do
    i = Factory(:item, :price => 3.987)
    i.price.should == 3.99
  end
end

line_item.rb

class LineItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :item
end

Spec results

Failures:

  1) Item 
     Failure/Error: it {should have_many(:orders).through(:line_items)}
       Expected Item to have a has_many association called orders (Item does not have any relationship to line_items)
     # ./spec/models/item_spec.rb:4