Filter out cart orders when fetching customers

We only care about non-cart orders and skipping carts, saves PostgreSQL
query planner to go through thousands of records in production use cases
(my food hub).

We go from

```sql
 ->  Index Scan using index_spree_orders_on_customer_id on spree_orders  (cost=0.42..12049.45 rows=152002 width=15) (actual time=0.015..11.703 rows=13867 loops=1)
```

to

```sql
->  Index Scan using index_spree_orders_on_customer_id on spree_orders  (cost=0.42..12429.46 rows=10802 width=15) (actual time=0.025..17.705 rows=9954 loops=1)
```

app/services/customers_with_balance.rb

@@ -8,7 +8,7 @@ class CustomersWithBalance
   def query
     Customer.of(enterprise_id).
       includes(:bill_address, :ship_address, user: :credit_cards).
-      joins("LEFT JOIN spree_orders ON spree_orders.customer_id = customers.id").
+      joins(left_join_non_cart_orders).
       group("customers.id").
       select("customers.*").
       select(outstanding_balance)
@@ -29,4 +29,11 @@ class CustomersWithBalance
        ) AS balance_value
     SQL
   end
+
+  def left_join_non_cart_orders
+    <<-SQL.strip_heredoc
+      LEFT JOIN spree_orders ON spree_orders.customer_id = customers.id
+        AND spree_orders.state != 'cart'
+    SQL
+  end
 end

spec/controllers/admin/customers_controller_spec.rb

@@ -83,12 +83,12 @@ module Admin
 
               it 'includes the customer balance in the response' do
                 spree_get :index, params
-                expect(json_response.first["balance"]).to eq("$-10.00")
+                expect(json_response.first["balance"]).to eq("$0.00")
               end
             end
 
             context 'when the customer has an order with a void payment' do
-              let(:order) { create(:order, customer: customer) }
+              let(:order) { create(:order, customer: customer, state: 'complete') }
               let!(:line_item) { create(:line_item, order: order, price: 10.0) }
               let!(:payment) { create(:payment, order: order, amount: order.total) }
 

spec/features/admin/customers_spec.rb

@@ -103,7 +103,7 @@ feature 'Customers' do
             payment_total: 88,
             distributor: managed_distributor1,
             user: nil,
-            completed_at: Time.zone.now,
+            state: 'complete',
             customer: customer1
           )
           create(
@@ -112,7 +112,7 @@ feature 'Customers' do
             payment_total: 0,
             distributor: managed_distributor1,
             user: nil,
-            completed_at: Time.zone.now,
+            state: 'complete',
             customer: customer2
           )
           create(
@@ -121,7 +121,7 @@ feature 'Customers' do
             payment_total: 0,
             distributor: managed_distributor1,
             user: nil,
-            completed_at: Time.zone.now,
+            state: 'complete',
             customer: customer4
           )
 

spec/services/customers_with_balance_spec.rb

@@ -8,13 +8,30 @@ describe CustomersWithBalance do
   describe '#query' do
     let(:customer) { create(:customer) }
 
+    context 'when orders are in cart state' do
+      let(:total) { 200.00 }
+      let(:order_total) { 100.00 }
+
+      before do
+        create(:order, customer: customer, total: order_total, payment_total: 0, state: 'cart')
+        create(:order, customer: customer, total: order_total, payment_total: 0, state: 'cart')
+      end
+
+      it 'returns the customer balance' do
+        customer = customers_with_balance.query.first
+        expect(customer.balance_value).to eq(0)
+      end
+    end
+
     context 'when no orders where paid' do
       let(:total) { 200.00 }
       let(:order_total) { 100.00 }
 
       before do
-        create(:order, customer: customer, total: order_total, payment_total: 0)
-        create(:order, customer: customer, total: order_total, payment_total: 0)
+        order = create(:order, customer: customer, total: order_total, payment_total: 0)
+        order.update_attribute(:state, 'checkout')
+        order = create(:order, customer: customer, total: order_total, payment_total: 0)
+        order.update_attribute(:state, 'checkout')
       end
 
       it 'returns the customer balance' do
@@ -29,8 +46,10 @@ describe CustomersWithBalance do
       let(:payment_total) { order_total }
 
       before do
-        create(:order, customer: customer, total: order_total, payment_total: 0)
-        create(:order, customer: customer, total: order_total, payment_total: payment_total)
+        order = create(:order, customer: customer, total: order_total, payment_total: 0)
+        order.update_attribute(:state, 'checkout')
+        order = create(:order, customer: customer, total: order_total, payment_total: payment_total)
+        order.update_attribute(:state, 'checkout')
       end
 
       it 'returns the customer balance' do
@@ -46,7 +65,8 @@ describe CustomersWithBalance do
       let(:non_canceled_orders_total) { order_total }
 
       before do
-        create(:order, customer: customer, total: order_total, payment_total: 0)
+        order = create(:order, customer: customer, total: order_total, payment_total: 0)
+        order.update_attribute(:state, 'checkout')
         create(
           :order,
           customer: customer,