Add extra meta for orders in Woocommerce

I'm creating a custom plugin for my website.

In some part of this plugin I need to store extra meta in wp_postmeta for each orders.

I added this in my plugin's class:

add_action ('woocommerce_before_checkout_process', array( &$this, 'add_item_meta', 10, 2) );

And this is add_item_meta() function:

function add_item_meta( $item_id, $values ) {
  wc_add_order_item_meta($item_id, '_has_event', 'yes' );
}

This function is not complete, but nothing happens with this codes; I think I need to use another hook but I can't find a proper one.

Does anyone know anything about this?

I also have another problem with $item_id: this is woocommerce global variable but I can't see it in my plugin!

I mean I don't have access to this variable from my plugin or something like this!

The 2018 way:

Built on Guido W.P. answer you can use instead woocommerce_checkout_create_order action hook in a more lighter and effective version code (using WC 3+ CRUD methods):

add_action('woocommerce_checkout_create_order', 'before_checkout_create_order', 20, 2);
function before_checkout_create_order( $order, $data ) {
    $order->update_meta_data( '_custom_meta_key', 'value' );
}

Code goes in function.php file of your active child theme (or active theme).

Tested and works in WooCommerce 3+ (only).

SOME EXPLANATIONS:

The woocommerce_checkout_create_order action hook is just one step before saving the order data. See below in an extract of the WC_Checkout create_order() method (with both hooks):

/**
 * Action hook to adjust order before save.
 * @since 3.0.0
 */
do_action( 'woocommerce_checkout_create_order', $order, $data );

// Save the order.
$order_id = $order->save();

do_action( 'woocommerce_checkout_update_order_meta', $order_id, $data );

return $order_id;

Why using woocommerce_checkout_create_order instead?:

• Because You don't need to use $order = wc_get_order( $order_id ); as you already got $order as an argument in the hooked function.
• You don't need to use $order->save(); as this will be done just after anyway (see the source code)
• This hook has been released since WooCommerce version 3 and it's made for the same purpose, allowing to use all available WC_Order methods.

So this just works with a single line of code inside the function.

Building on Mo Saeedi answer, I believe this snippet is more in line with the new CRUD approach introduced by WooCommerce 3.0:

add_action('woocommerce_checkout_update_order_meta',function( $order_id, $posted ) {
    $order = wc_get_order( $order_id );
    $order->update_meta_data( 'my_custom_meta_key', 'my data' );
    $order->save();
} , 10, 2);

See also this threads on the WordPress forums:

• Oh CRUD! Custom Meta to order from cart
• What is the correct way to update meta fields in Woocommerce?

answer is: I should use woocommerce_checkout_update_order_meta for add_action and also i should simply use update_post_meta() to add extra meta to my order

    function add_item_meta( $order_id ) {
            //global $woocommerce;
            update_post_meta( $order_id, '_has_event', 'yes' );
        } 

The 2016 way:

add_action('woocommerce_checkout_update_order_meta',function( $order_id, $posted ) {  
    update_post_meta( $order_id, 'my_custom_meta_key', 'my data' );  
} , 10, 2);

• $order_id is the id of the order, which is stored as a custom post type
• $posted is all the data from $_POST