Refactor DB query to aggregate customer balance

It's simpler and many orders of magnitude more efficient to ask the DB
to aggregate the customer balance based on their orders. It removes
a nasty N+1.

The resulting SQL query is:

```sql
SELECT customers.*, SUM(spree_orders.total - spree_orders.payment_total) AS balance
FROM "customers"
INNER JOIN "spree_orders"
    ON "spree_orders"."customer_id" = "customers"."id"
WHERE "customers"."enterprise_id" = 1
    AND (completed_at IS NOT NULL)
    AND (state != 'canceled')
GROUP BY customers.id
ORDER BY email;
```

app/controllers/admin/customers_controller.rb

@@ -66,7 +66,12 @@ module Admin
       return Customer.where("1=0") unless json_request? && params[:enterprise_id].present?
 
       Customer.of(managed_enterprise_id).
-        includes(:bill_address, :ship_address, user: :credit_cards)
+        includes(:bill_address, :ship_address, user: :credit_cards).
+        joins(:orders).
+        merge(Spree::Order.complete.not_state(:canceled)).
+        group("customers.id").
+        select("customers.*").
+        select("SUM(total - payment_total) AS balance_value")
     end
 
     def managed_enterprise_id

app/serializers/api/admin/customer_serializer.rb

@@ -9,6 +9,8 @@ module Api
       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
@@ -51,11 +53,6 @@ module Api
 
         options[:customer_tags].andand[object.id] || []
       end
-
-      def balance_value
-        @balance_value ||=
-          OpenFoodNetwork::UserBalanceCalculator.new(object.email, object.enterprise).balance
-      end
     end
   end
 end

spec/features/admin/customers_spec.rb

@@ -97,9 +97,9 @@ feature 'Customers' do
 
       describe "for a shop with multiple customers" do
         before do
-          mock_balance(customer1.email, managed_distributor1, 88)
-          mock_balance(customer2.email, managed_distributor1, -99)
-          mock_balance(customer4.email, managed_distributor1, 0)
+          build_balance(customer1, managed_distributor1, 88)
+          build_balance(customer2, managed_distributor1, -99)
+          build_balance(customer4, managed_distributor1, 0)
 
           customer4.update enterprise: managed_distributor1
         end
@@ -122,11 +122,12 @@ feature 'Customers' do
           end
         end
 
-        def mock_balance(email, enterprise, balance)
-          user_balance_calculator = instance_double(OpenFoodNetwork::UserBalanceCalculator)
-          expect(OpenFoodNetwork::UserBalanceCalculator).to receive(:new).
-            with(email, enterprise).and_return(user_balance_calculator)
-          expect(user_balance_calculator).to receive(:balance).and_return(balance)
+        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