I have two models Area and Unit.
Area has_many units and Unit belongs_to Area.
Generated scaffolds and modified to see a list of areas when editing Units.
When I create new units the area is saved without problems.
But when updating with another Area it keeps to original area_id although the newly chosen area id is written to the model parameters.

Here is the code from the edit action including logging calls printing the attributes of the model:
$this->_Logger->message("setting attributes",$this->Unit->getAttributes());
$this->Unit->setAttributes($this->params['Unit']);
$this->_Logger->message("set attributes",$this->Unit->getAttributes());
if($this->Request->isPost() && $this->Unit->save()){
$this->_Logger->message("setting attributes",$this->Unit->getAttributes());
$this->flash_options = array('seconds_to_close' => 10);
$this->flash['notice'] = $this->t('Unit was successfully '.($is_add?'created':'updated').'.');
$this->redirectTo(array('action' => 'show', 'id' => $this->Unit->getId()));
}

And here is the corresponding log:

Mon, 02 Feb 2009 02:30:28 +0100 [MESSAGE] setting attributes
PARAMS{
- id: 5
- area_id: 4
- name: Hall
- description:
- updated_at: 2009-02-02 02:24:38
- created_at: 2009-02-02 02:19:08
}
Mon, 02 Feb 2009 02:30:28 +0100 [MESSAGE] set attributes
PARAMS{
- id: 5
- area_id: 3
- name: Hall
- description:
- updated_at: 2009-02-02 02:24:38
- created_at: 2009-02-02 02:19:08
}
Mon, 02 Feb 2009 02:30:29 +0100 [MESSAGE] Updating Unit: UPDATE units SET id=5, area_id=4, name='Hall', description='', updated_at='2009-02-02 02:30:29', created_at='2009-02-02 02:19:08' WHERE id=5
Mon, 02 Feb 2009 02:30:29 +0100 [MESSAGE] saved attributes
PARAMS{
- id: 5
- area_id: 4
- name: Hall
- description:
- updated_at: 2009-02-02 02:30:29
- created_at: 2009-02-02 02:19:08
}

After setting the attibutes from the request to the model, the area_id changes to 3.
But the update SQL statement contains the old area_id = 4.

How am I supposed to handle the update ?
Do I need to update the new id through the association object before saving ?

Please advise.