[Migration] Make the callsign field in the table lotw_users unique

Make the callsign field in the table lotw_users unique to stop issue #2469 happening again

application/config/migration.php

@@ -22,7 +22,7 @@ $config['migration_enabled'] = TRUE;
 |
 */
 
-$config['migration_version'] = 141;
+$config['migration_version'] = 142;
 
 /*
 |--------------------------------------------------------------------------

application/migrations/142_lotw_users_call_unique.php

@@ -0,0 +1,16 @@
+<?php
+
+defined('BASEPATH') OR exit('No direct script access allowed');
+
+class Migration_lotw_users_call_unique extends CI_Migration {
+
+	public function up() {
+        // Update lotw_users and make the callsign column unique
+        $this->db->query("ALTER TABLE lotw_users ADD UNIQUE `callsign_index` (`callsign`);");
+    }
+
+	public function down(){
+         // Update lotw_users and make the callsign column not unique
+        $this->db->query("ALTER TABLE lotw_users DROP UNIQUE `callsign_index`;");
+	}
+}