Create new calculator from line item discounts

Set it as default in CreateItemAdjustment PromotionAction as the
previous FlatPercentItemTotal only works for discounts on orders

core/app/models/spree/calculator/percent_on_line_item.rb

@@ -0,0 +1,15 @@
+module Spree
+  class Calculator
+    class PercentOnLineItem < Calculator
+      preference :percent, :decimal, default: 0
+
+      def self.description
+        Spree.t(:percent_per_item)
+      end
+
+      def compute(object)
+        ((object.price * object.quantity) * preferred_percent) / 100
+      end
+    end
+  end
+end

core/app/models/spree/calculator/percent_per_item.rb

@@ -4,6 +4,9 @@ module Spree
   # for all matching products in an order. This should not be used as a
   # shipping calculator since it would be the same thing as a flat percent
   # off the entire order.
+  #
+  #
+  # TODO Should be deprecated now that we have adjustments at the line item level in spree core
 
   class Calculator::PercentPerItem < Calculator
     preference :percent, :decimal, default: 0

core/app/models/spree/promotion/actions/create_item_adjustment.rb

@@ -48,7 +48,7 @@ def promotion_credit_exists?(adjustable)
 
           def ensure_action_has_calculator
             return if self.calculator
-            self.calculator = Calculator::FlatPercentItemTotal.new
+            self.calculator = Calculator::PercentOnLineItem.new
           end
 
           def deals_with_adjustments

core/lib/spree/core/engine.rb

@@ -77,7 +77,7 @@ class Engine < ::Rails::Engine
 
         app.config.spree.calculators.add_class('promotion_actions_create_item_adjustments')
         app.config.spree.calculators.promotion_actions_create_item_adjustments = [
-          Spree::Calculator::FlatRate
+          Spree::Calculator::PercentOnLineItem
         ]
       end
 

core/spec/models/spree/calculator/percent_on_line_item_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+module Spree
+  class Calculator
+    describe PercentOnLineItem do
+      let(:line_item) { double("LineItem", price: 10, quantity: 10) }
+
+      before { subject.preferred_percent = 15 }
+
+      it "computes based on item price and quantity" do
+        expect(subject.compute(line_item)).to eq 15
+      end
+    end
+  end
+end