CakePHP add/edit form is working, except for foreign key references

I have a cakephp object (SecurityPerson) that has a few foreign key relationships to other objects.

Here's the add/edit view code that's generating the pulldowns for the foriegn-key references:

echo $form->input('ContractNumber');
echo $form->input('Location');

Here's the references to these objects in the model:

var $belongsTo = array(
	'ContractNumber' => array(
		'className' => 'ContractNumber',
		'foreignKey' => 'contract_number_id',
		'conditions' => '',
		'fields' => '',
		'order' => ''
	'Location' => array(
		'className' => 'Location',
		'foreignKey' => 'location_id',
		'conditions' => '',
		'fields' => '',
		'order' => ''

And here's the relevent part of the add() controller:

	if (!empty($this->data)) {
		if ($this->SecurityPerson->save($this->data)) {
			$this->Session->setFlash(__('The SecurityPerson has been saved', true));
		} else {
			$this->Session->setFlash(__('The SecurityPerson could not be saved. Please, try again.', true));
	$contractNumbers = $this->SecurityPerson->ContractNumber->find('list');
	$locations = $this->SecurityPerson->Location->find('list');
	$this->set(compact('contractNumbers', 'locations'));

It all looks good to me, including the generated HTML for the pulldowns on the add and edit pages. BUT... only nulls get added to the sercurity_person row.

Can anyone easily see what's going on, or give me a good start in troubleshooting?

EDIT: What I've tried so far:

I've tried looking at the SQL query. The field names for the foreign key ID's do not appear in the INSERT or UPDATE statement.

I've tried looking at the $data structure. The foreign key references look like this:
[ContractNumber] => 2
[Location] => 1