Extract balance-specific serializer

So we only show the customer balance where really needed. Aggregating
the balance can be costly. Also, we avoid defensive coding.

app/controllers/admin/customers_controller.rb

@@ -17,9 +17,10 @@ module Admin
       respond_to do |format|
         format.html
         format.json do
-          render_as_json @collection,
-                         tag_rule_mapping: tag_rule_mapping,
-                         customer_tags: customer_tags_by_id
+          render json: @collection,
+                 each_serializer: ::Api::Admin::CustomerWithBalanceSerializer,
+                 tag_rule_mapping: tag_rule_mapping,
+                 customer_tags: customer_tags_by_id
         end
       end
     end
@@ -63,9 +64,11 @@ module Admin
     private
 
     def collection
-      return Customer.where("1=0") unless json_request? && params[:enterprise_id].present?
-
-      CustomersWithBalance.new(managed_enterprise_id).query
+      if json_request? && params[:enterprise_id].present?
+        CustomersWithBalance.new(managed_enterprise_id).query
+      else
+        Customer.where('1=0')
+      end
     end
 
     def managed_enterprise_id

app/serializers/api/admin/customer_serializer.rb

@@ -4,13 +4,11 @@ module Api
   module Admin
     class CustomerSerializer < ActiveModel::Serializer
       attributes :id, :email, :enterprise_id, :user_id, :code, :tags, :tag_list, :name,
-                 :allow_charges, :default_card_present?, :balance, :balance_status
+                 :allow_charges, :default_card_present?
 
       has_one :ship_address, serializer: Api::AddressSerializer
       has_one :bill_address, serializer: Api::AddressSerializer
 
-      delegate :balance_value, to: :object
-
       def name
         object.name.presence || object.bill_address.andand.full_name
       end
@@ -19,20 +17,6 @@ module Api
         customer_tag_list.join(",")
       end
 
-      def balance
-        Spree::Money.new(balance_value, currency: Spree::Config[:currency]).to_s
-      end
-
-      def balance_status
-        if balance_value.positive?
-          "credit_owed"
-        elsif balance_value.negative?
-          "balance_due"
-        else
-          ""
-        end
-      end
-
       def tags
         customer_tag_list.map do |tag|
           tag_rule_map = options[:tag_rule_mapping].andand[tag]

app/serializers/api/admin/customer_with_balance_serializer.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module Api
+  module Admin
+    class CustomerWithBalanceSerializer < CustomerSerializer
+      attributes :balance, :balance_status
+
+      delegate :balance_value, to: :object
+
+      def balance
+        Spree::Money.new(balance_value, currency: Spree::Config[:currency]).to_s
+      end
+
+      def balance_status
+        if balance_value.positive?
+          "credit_owed"
+        elsif balance_value.negative?
+          "balance_due"
+        else
+          ""
+        end
+      end
+    end
+  end
+end

spec/controllers/admin/customers_controller_spec.rb

@@ -41,6 +41,11 @@ module Admin
               expect(ActiveModel::ArraySerializer).to receive(:new)
               spree_get :index, params
             end
+
+            it 'includes the customer balance in the response' do
+              spree_get :index, params
+              expect(json_response.first["balance"]).to eq("$0.00")
+            end
           end
 
           context "and enterprise_id is not given in params" do

spec/features/admin/customers_spec.rb

@@ -97,9 +97,33 @@ feature 'Customers' do
 
       describe "for a shop with multiple customers" do
         before do
-          build_balance(customer1, managed_distributor1, 88)
-          build_balance(customer2, managed_distributor1, -99)
-          build_balance(customer4, managed_distributor1, 0)
+          create(
+            :order,
+            total: 0,
+            payment_total: 88,
+            distributor: managed_distributor1,
+            user: nil,
+            completed_at: Time.zone.now,
+            customer: customer1
+          )
+          create(
+            :order,
+            total: 99,
+            payment_total: 0,
+            distributor: managed_distributor1,
+            user: nil,
+            completed_at: Time.zone.now,
+            customer: customer2
+          )
+          create(
+            :order,
+            total: 0,
+            payment_total: 0,
+            distributor: managed_distributor1,
+            user: nil,
+            completed_at: Time.zone.now,
+            customer: customer4
+          )
 
           customer4.update enterprise: managed_distributor1
         end
@@ -121,14 +145,6 @@ feature 'Customers' do
             expect(page).to have_content "$0.00"
           end
         end
-
-        def build_balance(customer, enterprise, balance)
-          order = build(:order, total: balance, payment_total: 0, distributor: enterprise)
-          order.email = customer.email
-          order.customer_id = customer.id
-          order.completed_at = Time.zone.now
-          order.save!
-        end
       end
 
       it "allows updating of attributes" do

spec/serializers/api/admin/customer_with_balance_serializer_spec.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Api::Admin::CustomerWithBalanceSerializer do
+  let(:serialized_customer) { described_class.new(customer) }
+
+  describe '#balance' do
+    let(:customer) { double(Customer, balance_value: 1.2) }
+    let(:money) { instance_double(Spree::Money, to_s: "$1.20") }
+
+    before do
+      allow(Spree::Money).to receive(:new).with(1.2, currency: "AUD") { money }
+    end
+
+    it 'returns the balance_value as a money amount' do
+      expect(serialized_customer.balance).to eq("$1.20")
+    end
+  end
+
+  describe '#balance_status' do
+    context 'when the balance_value is positive' do
+      let(:customer) { double(Customer, balance_value: 1) }
+
+      it 'returns credit_owed' do
+        expect(serialized_customer.balance_status).to eq("credit_owed")
+      end
+    end
+
+    context 'when the balance_value is negative' do
+      let(:customer) { double(Customer, balance_value: -1) }
+
+      it 'returns credit_owed' do
+        expect(serialized_customer.balance_status).to eq("balance_due")
+      end
+    end
+
+    context 'when the balance_value is zero' do
+      let(:customer) { double(Customer, balance_value: 0) }
+
+      it 'returns credit_owed' do
+        expect(serialized_customer.balance_status).to eq("")
+      end
+    end
+  end
+end