Abstract order balance in a simple PORO

This new class lets us [Branch by
abstraction](https://www.martinfowler.com/bliki/BranchByAbstraction.html)
by encapsulating an order's balance. As a result, that's the only place
where we need to check the feature toggle, instead of every place where
`#outstanding_balance` is called (quite some). That would be very hard
to review and it'd be more likely to introduce bugs.

What I like about this is that we also managed to group together the
data and logic that we had spread in a few places and have it nicely
encapsulated. So, where we had a number, we'll now have an object.

Once we fully change all `#outstanding_balance` consumers to use this
new abstraction we'll be able to remove the methods this class replaces.
These are: `Spree::Order#outstanding_balance?`,
`Spree::Order#display_oustanding_balance` and
`OrderHelper.outstanding_balance_label`.

This is just the first step. I'll follow this up with a PR per
page/mailer/whatever where we use the balance to replace it with an
instance of `OrderBalance`. That is, splitting up what I explored in
https://github.com/openfoodfoundation/openfoodnetwork/pull/6959 but in
very small and manageable pieces.

app/models/order_balance.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class OrderBalance
+  def initialize(order)
+    @order = order
+  end
+
+  def label
+    to_f.negative? ? I18n.t(:credit_owed) : I18n.t(:balance_due)
+  end
+
+  def amount
+    Spree::Money.new(to_f, currency: order.currency)
+  end
+
+  def to_f
+    if customer_balance_enabled?
+      order.new_outstanding_balance
+    else
+      order.outstanding_balance
+    end
+  end
+
+  delegate :zero?, to: :to_f
+
+  private
+
+  attr_reader :order
+
+  def customer_balance_enabled?
+    OpenFoodNetwork::FeatureToggle.enabled?(:customer_balance, order.user)
+  end
+end

spec/models/order_balance_spec.rb

@@ -0,0 +1,219 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe OrderBalance do
+  subject(:order_balance) { described_class.new(order) }
+  let(:order) { build(:order) }
+  let(:user) { order.user }
+
+  describe '#label' do
+    context 'when the customer_balance feature is disabled' do
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { false }
+      end
+
+      context 'when the balance is postive' do
+        before do
+          allow(order).to receive(:outstanding_balance) { 10 }
+        end
+
+        it "returns 'balance due'" do
+          expect(order_balance.label).to eq(I18n.t(:balance_due))
+        end
+      end
+
+      context 'when the balance is negative' do
+        before do
+          allow(order).to receive(:outstanding_balance) { -10 }
+        end
+
+        it "returns 'credit owed'" do
+          expect(order_balance.label).to eq(I18n.t(:credit_owed))
+        end
+      end
+
+      context 'when the balance is zero' do
+        before do
+          allow(order).to receive(:outstanding_balance) { 0 }
+        end
+
+        it "returns 'balance due'" do
+          expect(order_balance.label).to eq(I18n.t(:balance_due))
+        end
+      end
+    end
+
+    context 'when the customer_balance feature is enabled' do
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { true }
+      end
+
+      context 'when the balance is postive' do
+        before do
+          allow(order).to receive(:new_outstanding_balance) { 10 }
+        end
+
+        it "returns 'balance due'" do
+          expect(order_balance.label).to eq(I18n.t(:balance_due))
+        end
+      end
+
+      context 'when the balance is negative' do
+        before do
+          allow(order).to receive(:new_outstanding_balance) { -10 }
+        end
+
+        it "returns 'credit owed'" do
+          expect(order_balance.label).to eq(I18n.t(:credit_owed))
+        end
+      end
+
+      context 'when the balance is zero' do
+        before do
+          allow(order).to receive(:new_outstanding_balance) { 0 }
+        end
+
+        it "returns 'balance due'" do
+          expect(order_balance.label).to eq(I18n.t(:balance_due))
+        end
+      end
+    end
+  end
+
+  describe '#amount' do
+    context 'when the customer_balance feature is disabled' do
+      before do
+        allow(order).to receive(:outstanding_balance) { 10 }
+      end
+
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { false }
+      end
+
+      it 'returns the balance wraped in a Money object' do
+        expect(order_balance.amount).to eq(Spree::Money.new(10, currency: ENV['currency']))
+      end
+    end
+
+    context 'when the customer_balance feature is enabled' do
+      before do
+        allow(order).to receive(:new_outstanding_balance) { 20 }
+      end
+
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { true }
+      end
+
+      it 'returns the balance wraped in a Money object' do
+        expect(order_balance.amount).to eq(Spree::Money.new(20, currency: ENV['currency']))
+      end
+    end
+  end
+
+  describe '#zero?' do
+    context 'when the customer_balance feature is disabled' do
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { false }
+      end
+
+      context 'when the balance is zero' do
+        before do
+          allow(order).to receive(:outstanding_balance) { 0 }
+        end
+
+        it 'returns true' do
+          expect(order_balance.zero?).to eq(true)
+        end
+      end
+
+      context 'when the balance is positive' do
+        before do
+          allow(order).to receive(:outstanding_balance) { 10 }
+        end
+
+        it 'returns false' do
+          expect(order_balance.zero?).to eq(false)
+        end
+      end
+
+      context 'when the balance is negative' do
+        before do
+          allow(order).to receive(:outstanding_balance) { -10 }
+        end
+
+        it 'returns false' do
+          expect(order_balance.zero?).to eq(false)
+        end
+      end
+    end
+
+    context 'when the customer_balance feature is enabled' do
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { true }
+      end
+
+      context 'when the balance is zero' do
+        before do
+          allow(order).to receive(:new_outstanding_balance) { 0 }
+        end
+
+        it 'returns true' do
+          expect(order_balance.zero?).to eq(true)
+        end
+      end
+
+      context 'when the balance is positive' do
+        before do
+          allow(order).to receive(:new_outstanding_balance) { 10 }
+        end
+
+        it 'returns false' do
+          expect(order_balance.zero?).to eq(false)
+        end
+      end
+
+      context 'when the balance is negative' do
+        before do
+          allow(order).to receive(:new_outstanding_balance) { -10 }
+        end
+
+        it 'returns false' do
+          expect(order_balance.zero?).to eq(false)
+        end
+      end
+    end
+  end
+
+  describe '#to_f' do
+    context 'when the customer_balance feature is disabled' do
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { false }
+      end
+
+      it 'calls #outstanding_balance' do
+        expect(order).to receive(:outstanding_balance)
+        order_balance.to_f
+      end
+    end
+
+    context 'when the customer_balance feature is enabled' do
+      before do
+        allow(OpenFoodNetwork::FeatureToggle)
+          .to receive(:enabled?).with(:customer_balance, user) { true }
+      end
+
+      it 'calls #new_outstanding_balance' do
+        expect(order).to receive(:new_outstanding_balance)
+        order_balance.to_f
+      end
+    end
+  end
+end