Add itemwise shipping cost calculator

app/models/open_food_web/calculator/itemwise.rb

@@ -0,0 +1,13 @@
+module OpenFoodWeb
+  class Calculator::Itemwise < Spree::Calculator
+
+    def self.description
+      "Itemwise Shipping"
+    end
+
+    def compute(object)
+      # Given an order, sum the shipping on each individual item
+      object.line_items.map { |li| li.itemwise_shipping_cost }.inject(:+)
+    end
+  end
+end

config/application.rb

@@ -24,6 +24,12 @@ module Openfoodweb
       end
     end
 
+    # Register Spree calculators
+    initializer "spree.register.calculators" do |app|
+      app.config.spree.calculators.shipping_methods << OpenFoodWeb::Calculator::Itemwise
+    end
+
+
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.

spec/models/calculator/itemwise_spec.rb

@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+describe OpenFoodWeb::Calculator::Itemwise do
+  it "computes the shipping cost for each line item in an order" do
+    line_item = double(:line_item)
+    line_item.should_receive(:itemwise_shipping_cost).exactly(3).times.and_return(10)
+
+    order = double(:order)
+    order.stub(:line_items).and_return([line_item]*3)
+
+    subject.compute(order).should == 30
+  end
+end