Distributed Amount Calculator

backend/app/views/spree/admin/promotions/actions/_create_item_adjustments.html.erb

@@ -1,2 +1,6 @@
-<%= render partial: 'spree/admin/promotions/actions/promotion_calculators_with_custom_fields',
-           locals: { calculators: Spree::Promotion::Actions::CreateItemAdjustments.calculators, promotion_action: promotion_action, param_prefix: param_prefix } %>
+<%= render(
+  "spree/admin/promotions/actions/promotion_calculators_with_custom_fields",
+  calculators: Spree::Promotion::Actions::CreateItemAdjustments.calculators,
+  promotion_action: promotion_action,
+  param_prefix: param_prefix
+) %>

backend/app/views/spree/admin/promotions/calculators/weighted_rate/_fields.html.erb

@@ -0,0 +1,54 @@
+<%= fields_for "#{prefix}[calculator_attributes]", calculator do |f| %>
+  <%= f.label :preferred_amount %>
+  <%= render "spree/admin/shared/number_with_currency", f: f, amount_attr: :preferred_amount, currency_attr: :preferred_currency %>
+<% end %>
+
+<div class="field">
+  <p>
+    <%= admin_hint(
+      calculator.model_name.human,
+      """
+        <p>
+          This amount will be distributed to line items weighted relative to
+          their price. More expensive line items will receive a greater share
+          of the adjustment.
+        </p>
+
+        <p>
+          For example, with three line items and a preferred amount of $15 we
+          would end up with the following distribution:
+        </p>
+
+        <table>
+          <thead>
+            <tr>
+              <th></th>
+              <th>Price</th>
+              <th>Weighted Adj.</th>
+            </tr>
+          </thead>
+          <tbody>
+            <tr>
+              <td>Socks</td>
+              <td>$5</td>
+              <td>-$1.5</td>
+            </tr>
+            <tr>
+              <td>Shoes</td>
+              <td>$30</td>
+              <td>-$9</td>
+            </tr>
+            <tr>
+              <td>Slippers</td>
+              <td>$15</td>
+              <td>-$4.5</td>
+            </tr>
+          </tbody>
+        </table>
+      """
+    ) %>
+
+    This amount will be the <strong>total</strong> discount spread amongst all
+    of the line items.
+  </p>
+</div>

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

@@ -0,0 +1,24 @@
+require_dependency 'spree/calculator'
+
+# This is a calculator for line item adjustment actions. It accepts a line item
+# and calculates its weighted adjustment amount based on the value of the
+# preferred amount and the price of the other line items. More expensive line
+# items will receive a greater share of the preferred amount.
+
+module Spree
+  class Calculator::DistributedAmount < Calculator
+    preference :amount, :decimal, default: 0
+    preference :currency, :string, default: -> { Spree::Config[:currency] }
+
+    def compute_line_item(line_item)
+      if line_item && preferred_currency.casecmp(line_item.currency).zero?
+        Spree::DistributedAmountsHandler.new(
+          line_item,
+          preferred_amount
+        ).amount
+      else
+        0
+      end
+    end
+  end
+end

core/app/models/spree/distributed_amounts_handler.rb

@@ -0,0 +1,43 @@
+module Spree
+  class DistributedAmountsHandler
+    attr_reader :line_item, :order, :total_amount
+
+    def initialize(line_item, total_amount)
+      @line_item = line_item
+      @order = line_item.order
+      @total_amount = total_amount
+    end
+
+    # @return [Float] the weighted adjustment for the initialized line item
+    def amount
+      distributed_amounts[@line_item.id].to_f
+    end
+
+    private
+
+    # @private
+    # @return [Hash<Integer, BigDecimal>] a hash of line item IDs and their
+    #   corresponding weighted adjustments
+    def distributed_amounts
+      remaining_amount = @total_amount
+
+      @order.line_items.each_with_index.map do |line_item, i|
+        if i == @order.line_items.length - 1
+          # If this is the last line item on the order we want to use the
+          # remaining preferred amount to ensure our total adjustment is what
+          # has been set as the preferred amount.
+          [line_item.id, remaining_amount]
+        else
+          # Calculate the weighted amount by getting this line item's share of
+          # the order's total and multiplying it with the preferred amount.
+          weighted_amount = ((line_item.amount / @order.item_total) * total_amount).round(2)
+
+          # Subtract this line item's weighted amount from the total.
+          remaining_amount -= weighted_amount
+
+          [line_item.id, weighted_amount]
+        end
+      end.to_h
+    end
+  end
+end

core/config/locales/en.yml

@@ -401,6 +401,9 @@ en:
       spree/calculator/default_tax:
         one: Default Tax
         other: Default Tax
+      spree/calculator/distributed_amount:
+        one: Distributed Amount
+        other: Distributed Amount
       spree/calculator/flat_percent_item_total:
         one: Flat Percent
         other: Flat Percent

core/lib/spree/core/engine.rb

@@ -67,9 +67,10 @@ class Engine < ::Rails::Engine
         ]
 
         app.config.spree.calculators.promotion_actions_create_item_adjustments = %w[
-          Spree::Calculator::PercentOnLineItem
+          Spree::Calculator::DistributedAmount
           Spree::Calculator::FlatRate
           Spree::Calculator::FlexiRate
+          Spree::Calculator::PercentOnLineItem
           Spree::Calculator::TieredPercent
         ]
 

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

@@ -0,0 +1,32 @@
+require 'spec_helper'
+require 'shared_examples/calculator_shared_examples'
+
+describe Spree::Calculator::DistributedAmount, type: :model do
+  describe "#compute_line_item" do
+    subject { calculator.compute_line_item(order.line_items.first) }
+
+    let(:calculator) { Spree::Calculator::DistributedAmount.new }
+
+    let(:order) do
+      FactoryGirl.create(
+        :order_with_line_items,
+        line_items_attributes: [{ price: 50 }, { price: 50 }, { price: 50 }]
+      )
+    end
+
+    before do
+      calculator.preferred_amount = 15
+      calculator.preferred_currency = currency
+    end
+
+    context "when the order currency matches the store's currency" do
+      let(:currency) { "USD" }
+      it { is_expected.to eq 5 }
+    end
+
+    context "when the order currency does not match the store's currency" do
+      let(:currency) { "CAD" }
+      it { is_expected.to eq 0 }
+    end
+  end
+end

core/spec/models/spree/distributed_amounts_handler_spec.rb

@@ -0,0 +1,79 @@
+require "spec_helper"
+
+RSpec.describe Spree::DistributedAmountsHandler, type: :model do
+  let(:order) do
+    FactoryGirl.create(
+      :order_with_line_items,
+      line_items_attributes: line_items_attributes
+    )
+  end
+
+  describe "#amount" do
+    let(:total_amount) { 15 }
+
+    subject { described_class.new(line_item, total_amount).amount }
+
+    context "when there is only one line item" do
+      let(:line_items_attributes) { [{ price: 100 }] }
+      let(:line_item) { order.line_items.first }
+
+      it "applies the entire amount to the line item" do
+        expect(subject).to eq(15)
+      end
+    end
+
+    context "when there are multiple line items" do
+      let(:line_items_attributes) do
+        [{ price: 50 }, { price: 50 }, { price: 50 }]
+      end
+
+      context "and the line items are equally priced" do
+        it "evenly distributes the total amount" do
+          expect(
+            [
+              described_class.new(order.line_items[0], total_amount).amount,
+              described_class.new(order.line_items[1], total_amount).amount,
+              described_class.new(order.line_items[2], total_amount).amount
+            ]
+          ).to eq(
+            [5, 5, 5]
+          )
+        end
+
+        context "and the total amount cannot be equally distributed" do
+          let(:total_amount) { 10 }
+
+          it "applies the remainder of the total amount to the last item" do
+            expect(
+              [
+                described_class.new(order.line_items[0], total_amount).amount,
+                described_class.new(order.line_items[1], total_amount).amount,
+                described_class.new(order.line_items[2], total_amount).amount
+              ]
+            ).to eq(
+              [3.33, 3.33, 3.34]
+            )
+          end
+        end
+      end
+
+      context "and the line items are not equally priced" do
+        let(:line_items_attributes) do
+          [{ price: 150 }, { price: 50 }, { price: 100 }]
+        end
+
+        it "distributes the total amount relative to the item's price" do
+          expect(
+            [
+              described_class.new(order.line_items[0], total_amount).amount,
+              described_class.new(order.line_items[1], total_amount).amount,
+              described_class.new(order.line_items[2], total_amount).amount
+            ]
+          ).to eq(
+            [7.5, 2.5, 5]
+          )
+        end
+      end
+    end
+  end
+end