wp_get_object_terms() to get a list of all the terms attached to all the posts in the current query

Question

How can I use wp_get_object_terms() to get a list of all the terms attached to all posts in the current query?

For example, for the current query I want to get an array of the terms from the "Alfa" taxonomy that are contained in all the queried posts.

wp_get_object_terms($wp_query, 'alfa');

But that only seems to be returning one item in the array...

I am doing this to build an array to cross check one taxonomy with another for a navigation menu, and am currently doing this with the following code but I think there must be a better way.

Please help! Thanks!

$queried_terms = array();
if ( $wp_query->have_posts() ) : while ( $wp_query->have_posts() ) : $wp_query->the_post();
    $postid = $post->ID; 

    if( has_term( '', 'alfa', $postid) ) {
        $terms = get_the_terms( $postid, 'alfa' );
        foreach($terms as $term) {
            $queried_terms[] = $term->slug;
        }
    }

endwhile; endif;
rewind_posts();
wp_reset_query();

$queried_terms = array_unique($queried_terms);

Answer 1

I think you're on the right track because wp_get_object_terms() can take an array of IDs for its first argument, it's just that $wp_query is not the array you want.

I can't guarantee that this is more efficient (not my area of expertise), but I believe this [partially-tested] snippet would do what you want with at least one fewer loop and no array_unique():

// get $wp_query
global $wp_query;
// get array of post objects    
$my_posts = $wp_query -> posts;
// make array for the post IDs
$my_post_ids = array();
// loop through posts array for IDs
foreach( $my_posts as $my_post ) {
    $my_post_ids[] = $my_post->ID;
}
// get the terms
$my_terms = wp_get_object_terms( $my_post_ids, 'alfa' );

wp_get_object_terms() takes a 3rd $args parameter which you may need to set to get the ouput you want, but I'll leave that to you.

UPDATE: This can be even shorter using the new-to-me function wp_list_pluck(). Again this is untested but looks right:

// get $wp_query
global $wp_query;
// get array of post objects    
$my_posts = $wp_query -> posts;
// NEW: make array of the post IDs in one step
$my_post_ids = wp_list_pluck( $my_posts, 'ID' );
// get the terms
$my_terms = wp_get_object_terms( $my_post_ids, 'alfa' );

You can see in the source that this runs the same foreach loops code, but it looks a little nicer.

Answer 2

This generalisation of above worked for me:

$args = array( 'cat' = -1 ); // e.g. to get list of posts in any category
$postobjs = get_posts( $args );
$postids = wp_list_pluck( $postobjs );
$taxonomy = 'mytax' // your taxonomy name
$termobjs = wp_get_object_terms( $postids, $taxonomy );
$termlist = array_unique( wp_list_pluck( $termobjs, 'name' ) ); // distinct term names

It outputs a unique list of terms in the 'mytax' custom taxonomy. Thanks @mrwweb :-)