Activity form: once saved, can't select another mission

mod_form.php

@@ -100,4 +100,11 @@ class mod_labnbook_mod_form extends moodleform_mod {
         // Add standard buttons.
         $this->add_action_buttons();
     }
+
+    function definition_after_data() {
+        parent::definition_after_data();
+        if ($this->_form->getElementValue('update')) {
+            $this->_form->freeze('labnbook_missionid');
+        }
+    }
 }