Woocommerce get order key

If i understand correctly, you need get order_key by order_id, is it correct? If so, you can just use WC_Order property:

$test_order = new WC_Order($order_id);
$test_order_key = $test_order->order_key;

Edited

As mentioned indextwo, since Woo 3.0 there new syntax:

$test_order = wc_get_product($order_id);
$test_order_key = $test_order->get_order_key();

2018 updated answer

As WooCommerce 3 changed how property calls were made, the appropriate way to get the same information is:

$order = wc_get_order($order_id);

// Added a check to make sure it's a real order

if ($order && !is_wp_error($order)) {
    $order_key = $order->get_order_key();
}

Note that you can easily do the same in reverse: get an order ID from an order key:

$order_id = wc_get_order_id_by_order_key($order_key);