Weight calculator will calculate against a single line item as well as an order

app/models/open_food_web/calculator/weight.rb

@@ -8,8 +8,22 @@ module OpenFoodWeb
     end
 
     def compute(object)
-      total_weight = object.line_items.inject(0) { |sum, li| sum + ((li.variant.andand.weight || 0) * li.quantity) }
+      line_items = line_items_for object
+      total_weight = line_items.sum { |li| ((li.variant.andand.weight || 0) * li.quantity) }
       total_weight * self.preferred_per_kg
     end
+
+
+    private
+
+    def line_items_for(object)
+      if object.respond_to? :line_items
+        object.line_items
+      elsif object.respond_to?(:variant) && object.respond_to?(:quantity)
+        [object]
+      else
+        raise "Unknown object type: #{object.inspect}"
+      end
+    end
   end
 end

spec/models/calculator/weight_spec.rb

@@ -15,4 +15,13 @@ describe OpenFoodWeb::Calculator::Weight do
     subject.set_preference(:per_kg, 10)
     subject.compute(order).should == (10*1 + 20*3) * 10
   end
+
+  it "computes shipping cost for a line item" do
+    variant = double(:variant, :weight => 10)
+
+    line_item = double(:line_item, :variant => variant, :quantity => 2)
+
+    subject.set_preference(:per_kg, 10)
+    subject.compute(line_item).should == 10*2 * 10
+  end
 end