Extract balance-related methods into module

This model concerns helps us put together this related methods. Although
it doesn't provide any encapsulation yet, it makes a bit easier to
consider them all next time we need to change this implementation
somehow. It's a bit of an illusion but it feels like we are making this
God object model a bit smaller.

It also gives more room for documentation that will aid future devs.

app/models/concerns/balance.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require 'active_support/concern'
+
+# Contains the methods to compute an order balance form the point of view of the enterprise and not
+# the individual shopper.
+module Balance
+  FINALIZED_NON_SUCCESSFUL_STATES = %w(canceled returned).freeze
+
+  # Returns the order balance by considering the total as money owed to the order distributor aka.
+  # the shop, and as a positive balance of said enterprise. If the customer pays it all, they
+  # distributor and customer are even.
+  #
+  # Note however, this is meant to be used only in the context of a single order object. When
+  # working with a collection of orders, such an index controller action, please consider using
+  # `app/queries/oustanding_balance.rb` instead so we avoid potential N+1s.
+  def outstanding_balance
+    if state.in?(FINALIZED_NON_SUCCESSFUL_STATES)
+      -payment_total
+    else
+      total - payment_total
+    end
+  end
+
+  def outstanding_balance?
+    !outstanding_balance.zero?
+  end
+
+  def display_outstanding_balance
+    Spree::Money.new(outstanding_balance, currency: currency)
+  end
+end

app/models/spree/order.rb

@@ -9,7 +9,9 @@ require 'concerns/order_shipment'
 module Spree
   class Order < ActiveRecord::Base
     prepend OrderShipment
+
     include Checkout
+    include Balance
 
     checkout_flow do
       go_to_state :address
@@ -136,7 +138,6 @@ module Spree
 
     # All the states an order can be in after completing the checkout
     FINALIZED_STATES = %w(complete canceled resumed awaiting_return returned).freeze
-    FINALIZED_NON_SUCCESSFUL_STATES = %w(canceled returned).freeze
 
     scope :finalized, -> { where(state: FINALIZED_STATES) }
 
@@ -173,10 +174,6 @@ module Spree
       self[:currency] || Spree::Config[:currency]
     end
 
-    def display_outstanding_balance
-      Spree::Money.new(outstanding_balance, currency: currency)
-    end
-
     def display_item_total
       Spree::Money.new(item_total, currency: currency)
     end
@@ -394,18 +391,6 @@ module Spree
       Spree::TaxRate.adjust(self)
     end
 
-    def outstanding_balance
-      if state.in?(FINALIZED_NON_SUCCESSFUL_STATES)
-        -payment_total
-      else
-        total - payment_total
-      end
-    end
-
-    def outstanding_balance?
-      outstanding_balance != 0
-    end
-
     def name
       address = bill_address || ship_address
       return unless address

app/queries/outstanding_balance.rb

@@ -8,6 +8,9 @@
 # cases.
 #
 # See CompleteOrdersWithBalance or CustomersWithBalance as examples.
+#
+# Note this query object and `app/models/concerns/balance.rb` should implement the same behavior
+# until we find a better way. If you change one, please, change the other too.
 class OutstandingBalance
   # All the states of a finished order but that shouldn't count towards the balance (the customer
   # didn't get the order for whatever reason). Note it does not include complete

spec/models/concerns/balance_spec.rb

@@ -1,7 +1,6 @@
 require 'spec_helper'
 
-describe Spree::Order do
-
+describe Balance do
   context "#outstanding_balance" do
     context 'when orders are in cart state' do
       let(:order) { build(:order, total: 100, payment_total: 10, state: 'cart') }