CakePHP function which validate foreign keys

validateForeignKey

/**
 * Check if a given foreign key exists in the model (given its name).
 */
public function validateForeignKey($field){
	$associations = array_map(
		create_function('$v', 'return $v["foreignKey"];'),
		$this->belongsTo
	);
                                                                              
	$aliases = array();
	foreach($associations as $model => $foreignKey) {
		if (!array_key_exists($foreignKey, $aliases))
			$aliases[$foreignKey] = array();
		array_push($aliases[$foreignKey], $model);
	}
                                                                              
	foreach ($aliases[key($field)] as $model) {
		$count = $this->{$model}->find('count', array(
			'conditions' => array(
				$model . '.' . $this->{$model}->primaryKey => current($field)
			),
			'recursive' => -1
		));
		if ($count == 1)
			return true;
	}
	return false;
}

'User' should be replaced with $Model->alias

Plus here's how to use this rule in the model:

public $validate = array(
	'user_id' => array(
		'validateForeignKey' => array(
			'rule' => array('validateForeignKey', 'User'),
			'message' => 'Please enter a valid user.'
		),
	),
);

This works in cakephp 1.3.