public function constraints() {
		$constraints = array();
		$config  = $this->_config;
		$relFrom = $config['from']::meta('name');
		$relTo   = $config['name'];

		foreach ($this->_config['key'] as $from => $to) {
			$constraints["{$relFrom}.{$from}"] = "{$relTo}.{$to}";
		}
		return $constraints + (array) $this->_config['constraint'];
	}