Add by-weight shipping calculator

app/models/open_food_web/calculator/weight.rb

@@ -0,0 +1,15 @@
+module OpenFoodWeb
+  class Calculator::Weight < Spree::Calculator
+    preference :per_kg, :decimal, :default => 0.0
+    attr_accessible :preferred_per_kg
+
+    def self.description
+      "Weight (per kg)"
+    end
+
+    def compute(object)
+      total_weight = object.line_items.inject(0) { |sum, li| sum+li.variant.weight }
+      total_weight * self.preferred_per_kg
+    end
+  end
+end

config/application.rb

@@ -27,6 +27,7 @@ module Openfoodweb
     # Register Spree calculators
     initializer "spree.register.calculators" do |app|
       app.config.spree.calculators.shipping_methods << OpenFoodWeb::Calculator::Itemwise
+      app.config.spree.calculators.shipping_methods << OpenFoodWeb::Calculator::Weight
     end
 
 

spec/models/calculator/weight_spec.rb

@@ -0,0 +1,16 @@
+require 'spec_helper'
+
+describe OpenFoodWeb::Calculator::Weight do
+  it "computes shipping cost for an order by total weight" do
+    variant_1 = double(:variant, :weight => 10)
+    variant_2 = double(:variant, :weight => 20)
+
+    line_item_1 = double(:line_item, :variant => variant_1)
+    line_item_2 = double(:line_item, :variant => variant_2)
+
+    order = double(:order, :line_items => [line_item_1, line_item_2])
+
+    subject.set_preference(:per_kg, 10)
+    subject.compute(order).should == 300
+  end
+end