[phong-tran]

I’m newbie and my English is not good so the answer may be a little confusing.

Thank vlad274 for the advice.

My approach:

Same with Daniel Salcedo, I also try to change the metadata by editing woocommerce_sessions, but there is a problem. When adding an item to the cart, the woocommerce will create a unique cart_item_key for it by md5 (product_id + … + metadata), then look for this cart_item_key already exists, if available, this item will be updated in quantity, otherwise will create a new item.

This means that if you change the meta value of the hat product from blue to red, then add the hat product with blue, instead of creating a new item blue hat, it will only increase the quantity of red hat, you need to change the cart_item_key if you want the cart to be updated correctly.

Changing cart_item_key is quite risky, instead we can simply remove and re-add the product. Like this

// get cart_item_key of item you want to change
$cart_item_key_old = $_POST['cart_item_key']; 

// retrieve its information
$cart_item_old = WC()->cart->cart_contents[ $cart_item_key_old ];
$product_id_old = $cart_item_old['product_id'];
$quantity_old = $cart_item_old['quantity'];
$variation_id_old = $cart_item_old['variation_id'];
$variation_old = $cart_item_old['variation'];

// creating a cart_item_key with the same information except metadata
$cart_item_key_new = WC()->cart->generate_cart_id( $product_id_old, $variation_id_old, $variation_old, ['color'=>'red'] );
// check new cart_item_key already exists
$found = WC()->cart->find_product_in_cart( $cart_item_key_new );

// if true, update its quantity
if ($found != '') {
    $new_quantity = $cart_item_old['quantity'] + WC()->cart->cart_contents[ $cart_item_key_new ]['quantity'];
    WC()->cart->set_quantity( $cart_item_key_new, $new_quantity );
}
// else, re-add with new metadata
else {
    WC()->cart->add_to_cart($product_id_old, $quantity_old, $variation_id_old, $variation_old, ['color'=>'red'] );
}

// finally delete the old item
WC()->cart->remove_cart_item($cart_item_key_old);

wp_die();

Note: If you want to submit cart form instead of page refreshes after running the above ajax, the quantity of item is likely to be overridden by the set_quantity method when woocommerce update_cart. In this case you just need to return new_quantity and change the input value by js before submitting the form.

Full code:

foreach ( WC()->cart->get_cart() as $cart_item_key => $cart_item ) {
    ...
    ?>
    <!-- place it anywhere within the foreach -->
    <div class="box-type-field">
        <select class="box-type" name="box-type" cart_item_key="<?php echo $cart_item_key ?>">
            <option <?php echo $cart_item['box-type']=='boxes'?"selected":""; ?> value="boxes"><?php _e( 'Boxes', 'woocommerce' ); ?></option>
            <option <?php echo $cart_item['box-type']=='bags'?"selected":""; ?> value="bags"><?php _e( 'Bags', 'woocommerce' ); ?></option>
        </select>
    </div>
    <?php
    ...
}

AJAX:

$('.box-type-field .box-type').live('change', function () {
    var cartItemKey = $(this).attr("cart_item_key");
    var boxType = $(this).val();
    $.ajax({
        type : "post", 
        url : '<?php echo admin_url('admin-ajax.php');?>',
        datatype: 'json',
        data : {
            action : "update_cart_boxtype",
            cart_item_key : cartItemKey,
            box_type : boxType,
        },
        success: function(cartItem) {
            cartItemKey = cartItem[0];
            cartItemQty = cartItem[1];
            if (cartItem) $('input[name="cart['+cartItemKey+'][qty]"]').val(cartItemQty); // update quantity 

            $('.woocommerce-cart-form button[type="submit"]').click(); // submit form
        }
    })
})

PHP:

add_action( 'wp_ajax_update_cart_boxtype', 'update_cart_boxtype_init' );
add_action( 'wp_ajax_nopriv_update_cart_boxtype', 'update_cart_boxtype_init' );
function update_cart_boxtype_init() {
    if ( ! WC()->cart->is_empty() ) {
        $cart_item_key = (isset($_POST['cart_item_key']))?$_POST['cart_item_key'] : '';
        $cart_item = WC()->cart->cart_contents[ $cart_item_key ];
        $box_type = (isset($_POST['box_type']))?$_POST['box_type'] : '';
        $cart_updated = false;

        $cart_item_key_new = WC()->cart->generate_cart_id( $cart_item['product_id'], $cart_item['variation_id'], $cart_item['variation'], ['box-type'=>$box_type] );

        $found = WC()->cart->find_product_in_cart( $cart_item_key_new );

        if ($found != '') {
            $new_qty = $cart_item['quantity'] + WC()->cart->cart_contents[ $cart_item_key_new ]['quantity'];
            WC()->cart->remove_cart_item($cart_item_key);
            wp_send_json_success([$cart_item_key_new, $new_qty]);
        } else {
            WC()->cart->add_to_cart($cart_item['product_id'], $cart_item['quantity'], $cart_item['variation_id'], $cart_item['variation'], ['box-type' => $box_type]);
            $cart_updated = true;
            WC()->cart->remove_cart_item($cart_item_key);
            wp_send_json_success(false);
        }
    }
    wp_die();
}

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