WooCommerce add upsell product to the cart when product is added

I’m trying to implement a custom upsell function into my WooCommerce Theme.
When an user add a product, I need to add extra products (upsell) to its cart. That mean I have to add an extra parameter to the product which allow me to get the relation product/upsells and delete associated upsells if the product is deleted from the cart.

The user can choose these upsell products via a list of checkbox. Here is an overview of my “content-single-product.php” WooCommerce template:

<form action="<?php echo esc_url( $product->add_to_cart_url() ); ?>" class="cart" method="post" enctype="multipart/form-data">
    <?php

    // Get Upsells Product ID:
    $upsells = $product->get_upsells();

    if ( sizeof( $upsells ) == 0 ) return;

    $meta_query = $woocommerce->query->get_meta_query();

    // Get Upsells:
    $args = array(
        'post_type'           => 'product',
        'ignore_sticky_posts' => 1,
        'no_found_rows'       => 1,
        'posts_per_page'      => $posts_per_page,
        'orderby'             => $orderby,
        'post__in'            => $upsells,
        'post__not_in'        => array( $product->id )
    );

    $query = new WP_Query( $args );

    if ( $query->have_posts() ): ?>
        <ul>
        <?php while ( $query->have_posts() ): $query->the_post(); ?>
            <label for="upsell_<?php the_ID(); ?>"><input type="checkbox" name="upsells[]" id="upsell_<?php the_ID(); ?>" value="<?php the_ID(); ?>" /> <?php the_title(); ?></label>
        <?php endwhile; ?>
        </ul>
    <?php endif; ?>

    <?php wp_reset_postdata(); ?>

    <button type="submit">Add to cart</button>
</form>

The code above has been simplified..

Then, here is the code I added into functions.php which allow me to add upsells to the cart and add an extra row to the associated product:

/**
 * Add upsells as extra data for added product
 */
function add_upsells_to_cart( $cart_item_key ) {
    global $woocommerce;

    if ( empty( $_REQUEST['upsells'] ) || ! is_array( $_REQUEST['upsells'] ) )
        return;

    // Prevent loop
    $upsells = $_REQUEST['upsells'];
    unset( $_REQUEST['upsells'] );

    // Add upsell_parent row (if not already existing) for the associated product
    if ( ! $cart_item_data['upsells'] || ! is_array( $cart_item_data['upsells'] ) )
        $cart_item_data['upsells'] = array();

    // Append each upsells to product in cart
    foreach( $upsells as $upsell_id ) {
        $upsell_id = absint( $upsell_id );

        // Add extra parameter to the product which contain the upsells
        $woocommerce->cart->cart_contents[ $cart_item_key ]['upsells'] = $upsell_id;

        // Add upsell to cart
        $woocommerce->cart->add_to_cart( $upsell_id );
    }
}
add_action( 'woocommerce_add_to_cart', 'add_upsells_to_cart' );

Upsells are added to the cart as expected, but the extra row seems to be dropped by something in WC.

When I do a print_r( $woocommerce->cart->cart_contents[ $cart_item_key ] ); at the end of the previous function, the extra upsells parameter is visible, but when I display the cart from the cart page, the upsells parameter is dropped from the cart.

Any idea?

Thanks,
E.