Add a custom multi-select field to admin product options settings in Woocommerce

Question

I have followed this answer How to add more custom field in Linked Product of Woocommerce to add a custom select field to my Linked Product screen in Woocommerce. This new field

Answer 1

This kind of select field only works with a defined multiple attribute and work with an array of values. so you can't use it for a simple ID. If you add to your select field multiple="multiple" attribute it will work.

Also since Woocommerce 3 things have changed:
- There are better hooks to save the data.
- You can now use CRUD Objects and related methods.

The following code will work for multiple product IDs (an array of products IDs):

// Display a custom select field in "Linked Products" section
add_action( 'woocommerce_product_options_related', 'display_linked_products_data_custom_field' );
function display_linked_products_data_custom_field() {
    global $product_object, $post;
    ?>
    <p class="form-field">
        <label for="subscription_toggle_products"><?php _e( 'Subscription Toggle Products', 'woocommerce' ); ?></label>
        <select class="wc-product-search" multiple="multiple" style="width: 50%;" id="subscription_toggle_ids" name="_subscription_toggle_ids[]" data-placeholder="<?php esc_attr_e( 'Search for a product&hellip;', 'woocommerce' ); ?>" data-action="woocommerce_json_search_products_and_variations" data-exclude="<?php echo intval( $post->ID ); ?>">
            <?php
                $product_ids = $product_object->get_meta( '_subscription_toggle_ids' );

                foreach ( $product_ids as $product_id ) {
                    $product = wc_get_product( $product_id );
                    if ( is_object( $product ) ) {
                        echo '<option value="' . esc_attr( $product_id ) . '"' . selected( true, true, false ) . '>' . wp_kses_post( $product->get_formatted_name() ) . '</option>';
                    }
                }
            ?>
        </select>
    </p>
    <?php
}

// Save the values to the product
add_action( 'woocommerce_admin_process_product_object', 'save_linked_products_data_custom_field_value', 10, 1 );
function save_linked_products_data_custom_field_value( $product ){
    $data = isset( $_POST['_subscription_toggle_ids'] ) ? array_map( 'intval', (array) $_POST['_subscription_toggle_ids'] ) : array();
    $product->update_meta_data( '_subscription_toggle_ids', $data );
}

Code goes in function.php file of your active child theme (active theme). Tested and works.