Get only parent terms from wp_get_post_terms

Question

I've got a custom post type that has hierarchical taxonomies set up. For an example, my post type of "project" has categories of

 A
   A.1
 B
 C

I'm trying to get the taxonomies displayed as classes on <li> items for each post, but I just want the top level parents shown. For the post I'm looking at, it is categorized as A.1 and C, but I'd like to return A and C.

I'm using 'parent' => 0 in the args, however its giving me A.1 and C. I have also tried using 'hide_empty' => 0 but that doesn't seem to help.

Here's my code:

 function project_get_item_classes($post_id = null) {
     if ($post_id === null)
         return;
     $_terms = wp_get_post_terms($post_id, 'construction_type', array( 'parent' => 0, 'hide_empty' => 0 ));
     foreach ($_terms as $_term) {
         echo " " . $_term->slug;
     }
 }

Answer 1

wp_get_post_terms doesn't accept 'parent' or 'hide_empty' parameters in it's arguments array, only 'orderby','order' and 'fields' but you are on the right track, simply add a conditional check before you echo out the slug:

function project_get_item_classes($post_id = null) {
     if ($post_id === null)
         return;
     $_terms = wp_get_post_terms($post_id, 'construction_type');
     foreach ($_terms as $_term) {
        if ($_term->parent == 0) //check for parent terms only
            echo " " . $_term->slug;
     }
 }

Answer 2

Simply order by parent...

$terms = wp_get_post_terms(  $ID , $tax, array( 'orderby' => 'parent', 'order' => 'ASC' ) );

$myparent = $terms[0];

Answer 3

Thanks to help from @Bainternet (here, and also his input which led to the answer in this question), I was able to scrape together the code.

// determine the topmost parent of a term
function get_term_top_most_parent($term_id, $taxonomy){
    // start from the current term
    $parent  = get_term_by( 'id', $term_id, $taxonomy);
    // climb up the hierarchy until we reach a term with parent = '0'
    while ($parent->parent != '0'){
        $term_id = $parent->parent;

        $parent  = get_term_by( 'id', $term_id, $taxonomy);
    }
    return $parent;
}

// so once you have this function you can just loop over the results returned by wp_get_object_terms

function project_get_item_classes($taxonomy, $results = 1) {
    // get terms for current post
    $terms = wp_get_object_terms( get_the_ID(), 'work_type' );
    // set vars
    $top_parent_terms = array();
    foreach ( $terms as $term ) {
        //get top level parent
        $top_parent = get_term_top_most_parent( $term->term_id, 'work_type' );
        //check if you have it in your array to only add it once
        if ( !in_array( $top_parent, $top_parent_terms ) ) {
            $top_parent_terms[] = $top_parent;
        }
    }
    // build output (the HTML is up to you)

    foreach ( $top_parent_terms as $term ) {
        echo " " . $term->slug;
    }
}