change discount to percentage of matched product

app/models/spree/calculator/related_product_discount.rb

@@ -21,13 +21,11 @@ def compute(object)
 
         order.line_items.each do |li|
           if discount_applies_to.include? li.product
-            discount = relations.detect {|rel| rel.related_to == li.product}.discount_amount
-
-            total += if li.quantity < line_item.quantity
-              (discount * li.quantity)
-            else
-              (discount * line_item.quantity)
-            end
+            discount_percent = relations.detect {|rel| rel.related_to == li.product}.discount_amount
+
+            # only apply the discount as many times as the minimum quantity of matching products
+            quantity = [li.quantity, line_item.quantity].min
+            total += (li.price * discount_percent / 100.0) * quantity
           end
         end
 

app/models/spree/relation.rb

@@ -4,4 +4,5 @@ class Spree::Relation < ActiveRecord::Base
   belongs_to :related_to, :polymorphic => true
 
   validates_presence_of :relation_type, :relatable, :related_to
+  validates_inclusion_of :discount_amount, :in => 0..100
 end

app/views/spree/admin/products/_related_products_table.html.erb

@@ -10,8 +10,8 @@
   <tbody>
     <% product.relations.each do |relation| %>
       <tr id="<%= dom_id relation %>">
-        <td><%= relation.related_to.name %></td>
-        <td><%= relation.discount_amount != 0 ? number_to_currency(relation.discount_amount) : "-" %></td>
+        <td><%= link_to relation.related_to.name, relation.related_to %></td>
+        <td><%= relation.discount_amount != 0 ? number_to_percentage(relation.discount_amount, :precision => 0) : "none" %></td>
         <td><%= relation.relation_type.name %></td>
         <td width="70px">
           <%= link_to_delete relation, {:url => admin_product_relation_url(relation.relatable, relation)} %>

config/locales/en.yml

@@ -1,6 +1,6 @@
 en:
   new_relation_type: New Relation Type
-  discount_amount: Discount Amount
+  discount_amount: Discount %
   related_product_discount: Related Product Discount
   relation_types: Relation Types
   applies_to: Applies To