protected function _keys($keys) {
		if (!$keys) {
			return [];
		}
		$config = $this->_config;
		$hasType = ($config['type'] === 'hasOne' || $config['type'] === 'hasMany');
		$related = Libraries::locate('models', $config[$hasType ? 'from' : 'to']);

		if (!class_exists($related)) {
			throw new ClassNotFoundException("Related model class '{$related}' not found.");
		}
		if (!$related::key()) {
			throw new ConfigException("No key defined for related model `{$related}`.");
		}
		$keys = (array) $keys;
		$related = (array) $related::key();

		if (count($keys) !== count($related)) {
			$msg = "Unmatched keys in relationship `{$config['name']}` between models ";
			$msg .= "`{$config['from']}` and `{$config['to']}`.";
			throw new ConfigException($msg);
		}
		return $hasType ? array_combine($related, $keys) : array_combine($keys, $related);
	}