For Remi-Thank you

Hello Remi, thank you for great tutorial. I've got one question. I've set up my own code:
inside woo_add_custom_general_fields is
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echo '<div class="options_group">';
?>
<p class="form-field select_multiple_for_woocommerce">
<label for="select_multiple_for_woocommerce">My Select Field</label><select id="select_multiple_for_woocommerce" name="select_multiple_for_woocommerce[]" multiple="multiple" class="select short">
<option value="">Please select</option>
                        <?php global $product; $args = array(
          'post_type' => 'product',
'meta_key' =>'_featured'
        ); $productPosts = new WP_Query( $args ); ?>

                        <?php foreach ($productPosts->posts as $product): ?>
                            <option value="<?php echo $product->ID?>"
                                ><?php echo $product->post_title;?></option>
                        <?php endforeach; ?></select> </p>
<?php
echo '</div>';
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And function for save:
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function multiple_select_woo_add_backend_save( $post_id ){
$select_multiple_for_woocommerce =  $_POST['select_multiple_for_woocommerce'];
	update_post_meta( $post_id, 'select_multiple_for_woocommerce', $select_multiple_for_woocommerce );}
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Products are rendering good. However data is not being saved. Could you please take a look? Thank you :)