has_many :through (p108-110)

app/models/product.rb

@@ -1,6 +1,7 @@
 class Product < ActiveRecord::Base
   validates_presence_of :name, :description
   has_many :line_items
+  has_many :purchases, :through => :line_items
 
   before_validation :assign_default_description
 

app/models/purchase.rb

@@ -2,6 +2,7 @@ class Purchase < ActiveRecord::Base
   has_one :invoice
   belongs_to :supplier
   has_many :line_items
+  has_many :products, :through => :line_items
 
   def display_supplier
     return supplier.name unless supplier.nil?

app/views/products/show.html.erb

@@ -22,6 +22,13 @@
   <%= @product.stock %>
 </p>
 
+<h2>Related Purchases</h2>
+<ul>
+  <% @product.purchases.each do |purchase| %>
+    <li><%= link_to purchase.description, purchase %></li>
+  <% end %>
+</ul>
+
 <%= link_to 'Edit this record', edit_product_path(@product) %> <br />
 <%= link_to 'Delete this record', @product,
   { :confirm => 'Are you sure?', :method => :delete } %> <br />

app/views/purchases/show.html.erb

@@ -41,5 +41,12 @@
 
 <p><%= link_to "New Line Item", new_purchase_line_item_path(@purchase) %></p>
 
+<h2>Related Products</h2>
+<ul>
+  <% @purchase.products.each do |product| %>
+    <li><%= link_to product.name, product %></li>
+  <% end %>
+</ul>
+
 <%= link_to 'Edit', edit_purchase_path(@purchase) %> |
 <%= link_to 'Back', purchases_path %>