Instead of changing price subtract discount from order total.

This is a first step to allow us to apply discount differently to quantity
portions of the same variant. I.e. sell 3 first units for $10, the next three
for $9 and all the next for $8.

README.md

@@ -33,31 +33,52 @@ Cart Contents:
 
        Product                Quantity       Price       Total
        ----------------------------------------------------------------
-       Rails T-Shirt          1              20.99       19.99
+       Rails T-Shirt          1              19.99       19.99
+
+Order details:
+
+       Volume Discount:     0.00
+       Subtotal:           19.99
 
 ## Example 2
 
 Cart Contents:
 
        Product                Quantity       Price       Total
        ----------------------------------------------------------------
-       Rails T-Shirt          5              18.00       90.00
+       Rails T-Shirt          5              19.99       99.95
+
+Order details:
+
+       Volume Discount:    -9.95 # 5 * (19.99 - 18)
+       Subtotal:           90.00
 
 ## Example 3
 
 Cart Contents:
 
       Product                Quantity       Price       Total
       ----------------------------------------------------------------
-      Rails T-Shirt          6              18.00       108.00
+      Rails T-Shirt          6              19.99       119.94
+
+Order details:
+
+       Volume Discount:   -11.94 # 6 * (19.99 - 18)
+       Subtotal:          108.00
 
 ## Example 4
 
 Cart Contents:
 
       Product                Quantity       Price       Total
       ----------------------------------------------------------------
-      Rails T-Shirt          20             15.00       300.00
+      Rails T-Shirt          20             19.99       399.80
+
+Order details:
+
+       Volume Discount:   -99.80 # 20 * (19.99 - 15)
+       Subtotal:          300.00
+
 
 
 Why is it simple
@@ -78,6 +99,7 @@ The original extension had also some issues, such as defining overlapping
 ranges. The models were unnecessarily complicated. Why `acts_as_list` if you can
 just order volume prices by their lower quantity range end?
 
+
 Volume Customers
 ================
 
@@ -105,22 +127,33 @@ Assuming the same volume prices configuration as above. First order:
 
       Product                Quantity       Price       Total
       ----------------------------------------------------------------
-      Rails T-Shirt          8              18.00       144.00
+      Rails T-Shirt          8              19.99       159.92
+
+Order details:
+
+       Volume Discount:    -15.92 # 8 * (19.99 - 18)
+       Subtotal:           144.00
+
 
 Next order during the next 31 days:
 
       Product                Quantity       Price       Total
       ----------------------------------------------------------------
-      Rails T-Shirt          4              15.00       60.00
+      Rails T-Shirt          4              19.99       79.96
+
+Order details:
+
+       Volume Discount:     -7.96 # 4 * (19.99 - 18)
+       Subtotal:            72.00
 
 
 Additional Notes
 ================
 
-* The volume price is applied based on the total quantity ordered for
-  a particular variant. It does not (yet) apply different prices for the portion
-  of the quantity that falls within a particular range. Although I plan to
-  support such option.
+* The volume discount is calculated by applying the discount price to all
+  ordered units of a particular variant. It does not (yet) apply different
+  prices for the portion of the quantity that falls within a particular range.
+  Although I plan to support such option.
 
 Authors
 =======

app/models/line_item_decorator.rb

@@ -1,10 +1,31 @@
 LineItem.class_eval do
-  before_update :check_volume_pricing
+  before_save :check_update_volume_discount
 
-  private
-  def check_volume_pricing
-    if changed? && changes.keys.include?("quantity")
-      self.price = variant.volume_price quantity, order
+  def update_volume_discount updated_order = nil
+    self.volume_discount = 0
+
+    return if self.quantity < 1 || variant.volume_prices.empty?
+
+    self.order = updated_order if updated_order
+    total_quantity = self.quantity + order.variant_starting_quantity(variant)
+    final_price = default_price = self.price
+
+    variant.volume_prices.each do |vp|
+      break if vp.starting_quantity > total_quantity
+      final_price = vp.price
     end
+
+    self.volume_discount = self.quantity * (final_price - default_price)
+  end
+
+  def amount_with_volume_discount
+    update_volume_discount
+    amount_without_volume_discount + self.volume_discount
+  end
+  alias_method_chain :amount, :volume_discount
+
+  private
+  def check_update_volume_discount
+    update_volume_discount if price_changed? || quantity_changed?
   end
 end

app/models/order_decorator.rb

@@ -1,11 +1,7 @@
 Order.class_eval do
-  def add_variant_with_volume_prices variant, quantity = 1
-    current_item = add_variant_without_volume_prices variant, quantity
-    current_item.price = variant.volume_price current_item.quantity, self
-    raise ActiveRecord::RollBack unless current_item.save
-    update!
+  def volume_discount
+    line_items.map(&:volume_discount).sum
   end
-  alias_method_chain :add_variant, :volume_prices
 
   # By default volume price is calculated based only on quantity of the current
   # order. If you want to have "Volume Customers" - people who purchase at
@@ -19,14 +15,12 @@ def variant_starting_quantity variant
   end
 
   # This is required for Volume Customers
-  # It updates line items prices when user logs in
-  def recalculate_prices_on_user_association
-    if user_id_changed? || email_changed?
-      line_items.each do |li|
-        li.price = li.variant.volume_price li.quantity, self
-        li.save
-      end
-    end
+  # It updates item_total when user logs in
+  def update_totals_on_user_association
+    return unless user_id_changed? || email_changed?
+
+    line_items.each {|li| li.update_volume_discount self}
+    update_totals
   end
-  before_save :recalculate_prices_on_user_association
+  before_save :update_totals_on_user_association
 end

app/models/variant_decorator.rb

@@ -9,18 +9,6 @@
 
   after_create :copy_master_volume_prices
 
-  # calculates the price based on quantity
-  def volume_price quantity, order = nil
-    quantity += order.variant_starting_quantity self if order
-
-    price = self.price
-    volume_prices.each do |vp|
-      break if vp.starting_quantity > quantity
-      price = vp.price
-    end
-    price
-  end
-
   def blank_volume_price attributes
     attributes['starting_quantity'].blank? && attributes['price'].blank?
   end

app/views/order_mailer/cancel_email.text.erb

@@ -0,0 +1,17 @@
+Dear Customer,
+
+Your order has been CANCELED.  Please retain this cancellation information for your records.
+
+============================================================
+Order Summary [CANCELED]
+============================================================
+<% for item in @order.line_items %>
+<%=item.variant.sku %> <%=item.variant.product.name%> <%= variant_options(item.variant) %> (<%=item.quantity%>) @ <%= number_to_currency item.price %> = <%= number_to_currency(item.price * item.quantity) %>
+<% end %>
+============================================================
+Volume Discount: <%= number_to_currency @order.volume_discount %>
+Subtotal: <%= number_to_currency @order.item_total %>
+<% @order.adjustments.each do |adjustment| %>
+<%= "#{adjustment.label}: #{number_to_currency adjustment.amount}"%>
+<% end %>
+Order Total: <%= number_to_currency @order.total %>

app/views/order_mailer/confirm_email.text.erb

@@ -0,0 +1,20 @@
+Dear Customer,
+
+Please review and retain the following order information for your records.
+
+============================================================
+Order Summary
+============================================================
+<% for item in @order.line_items %>
+<%=item.variant.sku %> <%=item.variant.product.name%> <%= variant_options(item.variant) %> (<%=item.quantity%>) @ <%= number_to_currency item.price %> = <%= number_to_currency(item.price * item.quantity) %>
+<% end %>
+============================================================
+Volume Discount: <%= number_to_currency @order.volume_discount %>
+Subtotal: <%= number_to_currency @order.item_total %>
+<% @order.adjustments.each do |adjustment| %>
+<%= "#{adjustment.label}: #{number_to_currency adjustment.amount}"%>
+<% end %>
+Order Total: <%= number_to_currency @order.total %>
+
+
+Thank you for your business.

app/views/orders/_cart_volume_discount.html.erb

@@ -0,0 +1,18 @@
+<% if @order.volume_discount != 0.0 %>
+  <style>
+    #cart-volume-discount {
+      float: right;
+      width: 30%;
+      text-align: left;
+    }
+    div#subtotal {
+      clear: both;
+      width: 30%;
+    }
+  </style>
+  <div id="cart-volume-discount">
+    <h3>
+      <%= t 'volume_discount' %>: <%= format_price @order.volume_discount %>
+    </h3>
+  </div>
+<% end %>

app/views/shared/_order_details_volume_discount.html.erb

@@ -0,0 +1,8 @@
+<% if @order.volume_discount != 0.0 %>
+  <tbody id='volume-discount'>
+    <tr class="total" id="volume-discount-row">
+      <td colspan="3"><b><%= t('volume_discount') %>:</b></td>
+      <td class="total"><span><%= number_to_currency @order.volume_discount %></span></td>
+    </tr>
+  </tbody>
+<% end %>

config/locales/en.yml

@@ -1,5 +1,6 @@
 en:
   volume_pricing: Volume Pricing
   volume_prices: Volume Prices
+  volume_discount: Volume Discount
   starting_from: Starting from
   add_volume_price: Add Volume Price

db/migrate/20110302170148_add_volume_discount_to_line_items.rb

@@ -0,0 +1,10 @@
+class AddVolumeDiscountToLineItems < ActiveRecord::Migration
+  def self.up
+    add_column :line_items, :volume_discount, :decimal,
+                 :precision => 8, :scale => 2, :null => false, :default => 0
+  end
+
+  def self.down
+    remove_column :line_items, :volume_discount
+  end
+end

lib/volume_pricing_hooks.rb

@@ -2,4 +2,6 @@ class VolumePricingHooks < Spree::ThemeSupport::HookListener
   insert_after :admin_product_tabs, :partial => "admin/shared/vp_product_tab"
   insert_after :admin_variant_edit_form, :partial => "admin/variants/volume_prices"
   replace :product_price, :partial => "products/volume_prices"
+  insert_after :cart_items, :partial => "orders/cart_volume_discount"
+  insert_before :order_details_subtotal, :partial => "shared/order_details_volume_discount"
 end

spree_simple_volume_pricing.gemspec

@@ -3,7 +3,7 @@
 Gem::Specification.new do |s|
   s.platform = Gem::Platform::RUBY
   s.name     = 'spree_simple_volume_pricing'
-  s.version  = '1.0.0'
+  s.version  = '2.0.0'
   s.summary  = 'Adds volume pricing capabilities to Spree'
 
   s.author   = 'Adam Wróbel'