Backport StoreCredit#amount_remaining Solidus PR

This commit backports work from an open [Solidus PR][1]

[1]: solidusio#1512

This migration adds the `amount_remaining` column to the
`spree_store_credit_events` table, and iterates over existing Store
Credit Events to calculate historical data for this column. This value
is also displayed in the admin, instead of the User's total remaining
store credit.

api/app/helpers/spree/api/api_helpers.rb

@@ -174,7 +174,7 @@ def required_fields_for(model)
 
       @@store_credit_history_attributes = [
         :display_amount, :display_user_total_amount, :display_action,
-        :display_event_date
+        :display_event_date, :display_remaining_amount
       ]
 
       @@stock_transfer_attributes = [:id, :number]

backend/app/views/spree/admin/store_credits/show.html.erb

@@ -101,7 +101,7 @@
           <td><%= store_credit_event_admin_action_name(event) %></td>
           <td class='align-right'><%= event.display_amount %></td>
           <td><%= store_credit_event_originator_link(event) %></td>
-          <td class='align-right'><%= event.display_user_total_amount %></td>
+          <td class='align-right'><%= event.display_remaining_amount %></td>
           <td><%= event.update_reason.try!(:name) %></td>
         </tr>
       <% end %>

core/app/models/spree/store_credit.rb

@@ -250,6 +250,7 @@ def store_event
     event.update_attributes!({
       amount: action_amount || amount,
       authorization_code: action_authorization_code || event.authorization_code || generate_authorization_code,
+      amount_remaining: amount_remaining,
       user_total_amount: user.total_available_store_credit,
       originator: action_originator,
       update_reason: update_reason,

core/app/models/spree/store_credit_event.rb

@@ -38,6 +38,10 @@ def display_user_total_amount
       Spree::Money.new(user_total_amount, { currency: currency })
     end
 
+    def display_remaining_amount
+      Spree::Money.new(amount_remaining, { currency: currency })
+    end
+
     def display_event_date
       I18n.l(created_at.to_date, format: :long)
     end

core/db/migrate/20161006104057_add_amount_remaining_to_store_credit_events.rb

@@ -0,0 +1,37 @@
+class AddAmountRemainingToStoreCreditEvents < ActiveRecord::Migration
+  def up
+    add_column :spree_store_credit_events, :amount_remaining, :decimal, precision: 8, scale: 2, default: nil, null: true
+
+    Spree::StoreCredit.all.each do |credit|
+      credit_amount = credit.amount
+
+      credit.store_credit_events.chronological.each do |event|
+        case event.action
+        when Spree::StoreCredit::ALLOCATION_ACTION,
+             Spree::StoreCredit::ELIGIBLE_ACTION,
+             Spree::StoreCredit::CAPTURE_ACTION
+          # These actions do not change the amount_remaining so the previous
+          # amount available is used (either the credit's amount or the
+          # amount_remaining coming from the event right before this one).
+        when Spree::StoreCredit::AUTHORIZE_ACTION,
+             Spree::StoreCredit::INVALIDATE_ACTION
+          # These actions remove the amount from the available credit amount.
+          credit_amount -= event.amount
+        when Spree::StoreCredit::ADJUSTMENT_ACTION,
+             Spree::StoreCredit::CREDIT_ACTION,
+             Spree::StoreCredit::VOID_ACTION
+          # These actions add the amount to the available credit amount. For
+          # ADJUSTMENT_ACTION the event's amount could be negative (so it could
+          # end up subtracting the amount).
+          credit_amount += event.amount
+        end
+
+        event.update_attribute(:amount_remaining, credit_amount)
+      end
+    end
+  end
+
+  def down
+    remove_column :spree_store_credit_events, :amount_remaining
+  end
+end

core/spec/models/spree/store_credit_event_spec.rb

@@ -220,6 +220,20 @@
     end
   end
 
+  describe "#display_remaining_amount" do
+    let(:amount_remaining) { 300.0 }
+
+    subject { create(:store_credit_auth_event, amount_remaining: amount_remaining) }
+
+    it "returns a Spree::Money instance" do
+      expect(subject.display_remaining_amount).to be_instance_of(Spree::Money)
+    end
+
+    it "uses the events amount_remaining attribute" do
+      expect(subject.display_remaining_amount).to eq Spree::Money.new(amount_remaining, { currency: subject.currency })
+    end
+  end
+
   describe "#display_event_date" do
     let(:date) { DateTime.new(2014, 06, 1) }
 

core/spec/models/spree/store_credit_spec.rb

@@ -736,7 +736,7 @@
       end
     end
 
-    describe "#store_events" do
+    describe "#store_event" do
       context "create" do
         context "user has one store credit" do
           let(:store_credit_amount) { 100.0 }
@@ -754,19 +754,34 @@
           it "saves the user's total store credit in the event" do
             expect(subject.store_credit_events.first.user_total_amount).to eq store_credit_amount
           end
+
+          it "saves the user's unused store credit in the event" do
+            expect(subject.store_credit_events.first.amount_remaining).to eq store_credit_amount
+          end
         end
 
         context "user has multiple store credits" do
           let(:store_credit_amount)            { 100.0 }
           let(:additional_store_credit_amount) { 200.0 }
-
           let(:user)                           { create(:user) }
-          let!(:store_credit)                  { create(:store_credit, user: user, amount: store_credit_amount) }
 
-          subject { create(:store_credit, user: user, amount: additional_store_credit_amount) }
+          let!(:store_credits) do
+            [
+              create(:store_credit, user: user, amount: store_credit_amount),
+              create(:store_credit, user: user, amount: additional_store_credit_amount)
+            ]
+          end
+
+          subject { store_credits.flat_map(&:store_credit_events) }
 
           it "saves the user's total store credit in the event" do
-            expect(subject.store_credit_events.first.user_total_amount).to eq (store_credit_amount + additional_store_credit_amount)
+            expect(subject.first.user_total_amount).to eq store_credit_amount
+            expect(subject.last.user_total_amount).to eq(store_credit_amount + additional_store_credit_amount)
+          end
+
+          it "saves the user's unused store credit in the event" do
+            expect(subject.first.amount_remaining).to eq store_credit_amount
+            expect(subject.last.amount_remaining).to eq additional_store_credit_amount
           end
         end