Get all terms in a custom WordPress Query?

I know how to get all of the WordPress terms, but I need a “filtered” down version of the results. Is it possible to get all of the terms that are in the result of a WordPress query? I have this query here:

<?php
   $args=array(
  'post_type' => 'gw_activity',
  'post_status' => 'publish',
  'orderby' => 'date',
  'meta_query' => array(
     'relation' => 'AND',
      array(
         'key' => 'activity_category',
         'value' => 'mindful_life',
         'compare' => '='
      )
   ), 
  'posts_per_page' => 10
);
$my_query = null; 
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) { 
 $all_terms = array();
  while ($my_query->have_posts()) : $my_query->the_post(); ?>      
  <?php $terms = wp_get_post_terms( $my_query->post->ID, array( 'gw_activity_tag' ) ); ?>
  <?php       
        foreach ( $terms as $term ) {
            $all_terms[] = $term->name;
        }        
  ?>                                            
  <?php endwhile; }
  wp_reset_query();
?>
<!-- End Custom Query -->
<?php
    $unique_terms = array_unique( $all_terms ); 
    $result = array_unique($unique_terms);
    foreach ($result as $value) {
        echo $value . '<br />';
    }

?>

But I can’t figure how to run the query & put a “Where” clause in it, like you can with MySQL. Any help / suggestion or even point me in the right direction would be greatly appreciated. I’m stuck

Answer

Check this function: wp_get_post_terms()

Assuming your post supports two taxonomies called tax_a and tax_b, you can try something like this, exactly where you wrote your comment:

<?php $terms = wp_get_post_terms( $query->post->ID, array( 'tax_a', 'tax_b' ) ); ?>

<?php foreach ( $terms as $term ) : ?>
    <p><?php echo $term->taxonomy; ?>: <?php echo $term->name; ?></p>
<?php endforeach; ?>

This will print all the terms for each post retrieved by the query.

---

EDIT

If what you want is all the terms in all the posts retrieved by the query, you can store the values in an array and then use a function like array_unique(), like this:

$all_terms = array();
foreach ( $terms as $term ) {
    $all_terms[] = $term->name;
}

// ... and outside the WHILE loop
$result = array_unique( $all_terms );
foreach ( $result as $term ) {
    echo $term . '<br/>;
}