Create application.js

assets/javascripts/application.js

@@ -0,0 +1,9 @@
+$(function() {
+    $("select#issue_customer_id").on("change", function() {
+        $.ajax({
+            url:  "/filter_vehicles_by_customer",
+            type: "GET",
+            data: { selected_customer: $("select#issue_customer_id").val() }
+        });
+    });
+});