use rowCount

booking.variety_form.inc

@@ -147,10 +147,12 @@ function booking_variety_regn_form_validate($form, &$form_state) {
   // --- Check that this person hasn't already registered for variety sessions
 	$prev_regn_query = db_select('booking_variety_regn', 'v')
     ->condition('v.booking_person_nid', $values['booking_nid'], '=')
-    ->fields('v');
-	$prev_regn_check = $prev_regn_query->execute();
+    ->fields('v')
+    ->execute();
     
-  if ($prev_regn_check) {
+  watchdog('booking_debug', 'variety session registration rowcount: <pre>@info</pre>', array('@info' => $prev_regn_query->rowCount()));
+
+  if ($prev_regn_query->rowCount() > 0) {
     form_set_error('booking_nid', t('You have already registered for variety sessions. Existing registration cannot be changed.'));
     watchdog('booking_debug', 'Previous variety session registration: <pre>@info</pre>', array('@info' => print_r( $prev_regn_check, true)));
   }