Improve Spree::Order::NumberGenerator speed

When generating a number, if it already exists, the length of numbers
will be increased if over half of the possible options are already
taken. This query is very expensive when having millions of records.
Upon multiple number collisions, this SQL becomes a bottleneck.

Co-authored-by: Sung <[email protected]>

core/app/models/spree/order/number_generator.rb

@@ -35,11 +35,17 @@ def generate
         # Use the random number if no other order exists with it.
         if Spree::Order.exists?(number: random)
           # If over half of all possible options are taken add another digit.
-          @length += 1 if Spree::Order.count > (10**@length / 2)
+          @length += 1 if order_count > (10**@length / 2)
         else
           break random
         end
       end
     end
+
+    private
+
+    def order_count
+      @order_count ||= Spree::Order.count
+    end
   end
 end