How to add terms (without deleting others)

Question

I have a form on my site that allows signed in users tag products from the front-end of the site in a basic form. I am using Woocommerce as the commerce platform.

Woocommerce uses Custom Post Types to register its products. Product tags is a taxonomy (product_tag) Woocommerce uses for tagging products.

I am using wp_set_object_terms to insert the terms the users submit. Problem is when they submit new "tags" it overwrites any existing "Tags" and replaces with their new ones.

I just need a way for the form to add their tags and keep the existing tags of the product (post).

here is what I have:

if (is_user_logged_in()) {


    if(isset($_POST['submitted']) && isset($_POST['post_nonce_field']) && wp_verify_nonce($_POST['post_nonce_field'], 'post_nonce')) {


    //Tagging
    wp_set_object_terms( $post->ID, explode( ',', $_POST['postTags'] ), 'product_tag' );

}

?>
<form name="primaryTagForm" action="#" id="primaryTagForm" method="POST" enctype="multipart/form-data" >
    <fieldset class="tags">
        <label for="post_tags">Tags:</label>
        <input type="text" value="" tabindex="35" name="postTags" id="postTags" />
    </fieldset>

    <fieldset>
        <input type="hidden" name="submitted" id="submitted" value="true" />
        <?php wp_nonce_field( 'post_nonce', 'post_nonce_field' ); ?>
        <button class="button" type="submit"><?php _e('Tag Product', 'framework') ?></button>
    </fieldset>
</form>

Answer 1

The fourth parameter to wp_set_object_terms() is an "append" argument.

$append (bool) (required) If true, tags will be appended to the object. If false, tags will replace existing tags

    Default: False

http://codex.wordpress.org/Function_Reference/wp_set_object_terms

Your code should work if you pass true as that fourth argument.

wp_set_object_terms( 
  $post->ID, 
  explode( ',', $_POST['postTags'] ), 
  'product_tag',
  true
);