Reset database field values when active page is unloaded

In my app, I have a record locking mechanism that prevents two people from being in the Edit view of a record at the same time. Here is the function that locks the record when it is called up in the Edit view:

public function lockRecord($id = null) {
$this->loadModel('User');
$this->Model->id = $id;
$current = $this->Model->read(null, $id);

//Get the current logged-in user's ID
$userid = $current['Model']['requester_id'];

//Get the current lock expiry time
$lock_time = $this->Model->find('first', array(
  'fields'=>array('Model.lock_expiry_time'),
  'conditions'=>array('Model.id'=>$id)
  )
); 

//Get the ID of the user who has the record lock (if any)
$logged_user = $this->Model->find('first', array(
  'fields'=>array('Model.lock_key'),
  'conditions'=>array('Model.id'=>$id)
  )
); 

//Get that same user's full name
$full_name = $this->User->find('first', array(
  'joins' => array(
    array(
      'table' => 'recordtable',
      'alias' => 'Model',
      'type' => 'INNER',
      'conditions' => array('Model.lock_key = User.id')
      )
    )
  )
);
$this->set('lock_time', $lock_time);
$this->set(compact($current));
$this->set(compact($logged_user));
$this->set(compact($full_name)); 
if(AuthComponent::user('id') != $logged_user['Msr']['lock_key'] && date("Y-m-d H:i:s") < $lock_time['Msr']['lock_expiry_time']) {
  $this->Session->setFlash(__('This MSR is locked for editing by ' . $full_name['User']['full_name'] . '. Please try again in a few minutes or wait for this user to close the document.<br/>
    (Lock expires at '. $lock_time['Msr']['lock_expiry_time'] . ')'));
  $this->redirect(array('action' => 'view', $id));
} else {
  //Set a new lock key and expiry time if the record is free for editing
  $locksession = $this->Msr->query("UPDATE msrs SET lock_key = {$userid}, lock_expiry_time = ADDTIME(NOW(), '00:05:00') WHERE id = {$id}");
  $this->set('locksession', $locksession);
  }
}

When the user saves their changes, the unlockRecord function is called to release the key, reset the lock_expiry_time, and redirect the user to the "View" view:

public function unlockRecord($id = null) {
//Get a list of security groups
$groups = $this->Session->read('groups');
$this->Msr->id = $id;
//Reset the lock_key and the lock_expiry_time
$locksession = $this->Msr->query("UPDATE msrs SET lock_key = '', lock_expiry_time = '' WHERE id = {$id} ");
//If the module admin manually releases the lock, display a message
if(in_array('msr_module_admin', $groups)) {
  $this->Session->setFlash(__('The MSR has been unlocked and is available for editing.'));
}
$this->redirect(array('action'=>'view', $id));
}

There are three other conditions where the lock can be released:

--An administrator manually releases the lock by clicking a link.

--The user logs out of the system entirely.

--The lock expires five minutes after it is set.

I need the lock to be released any time the record in the Edit view is no longer active. For instance, if the user clicks off to another site or clicks a different link in my own site; anything that takes them away from the record they have open in the Edit view. lock_key should be set to '' and lock_expiry_time set to ''. How might I accomplish this?