List of post categories only associated to another custom taxonomy

Question

I've got a custom taxonomy "sector" associated to posts; posts are assigned to the default categories as well. What I'd like to achieve is to show in a custom sidebar a list of "categories" link/name, the post of which are assigned to the selected "sector".

Eg.: say that my data are organized like this (hope this make sense), with three posts associated to a various categories AND to a particular sector:

Post 1

• category: category_1
• sector: sector_1

Post 2

• category: category_2
• sector: sector_1

Post 3

• category: category_3
• sector: sector_1

Now, when I'm in the "sector_1" archive page, in the sidebar I'd like to show the following list:

• category_1
• category_2
• category_3

Is this possible? Maybe using tax_query or other methods?

Answer 1

Here's what I come up with, looks like it's working. But maybe (surely) there's a more efficient way to do the same:

// current queried
$term = 'sector';
$current_sector_id = get_queried_object_id();

// Prepare wp_query to get all posts assigned to the current "sector" 
$sector_posts_query = new WP_Query();
$sector_posts_query->query(
    array(
        'post_type' => 'post',
        'posts_per_page' => -1,
        'tax_query' => array(
            array(
                'taxonomy' => $term,
                'field' => 'term_id',
                'terms' => $current_sector_id
            ),
        )
    )
);

// loop the posts
if ($sector_posts_query->have_posts()) {
    $terms_array = array();
    while ($sector_posts_query->have_posts()) : $sector_posts_query->the_post();
        // get the product_cat of each post and store it as arrays in the $terms_array
        $current_category_terms = get_the_terms(get_the_ID(), 'my_category');
        $terms_array[] = wp_list_pluck($current_category_terms, 'term_id');
    endwhile;
    wp_reset_postdata();
}
// merge the inner arrays and remove doubles
$result = call_user_func_array("array_merge", $terms_array);
$result = array_unique($result, SORT_REGULAR);

echo '<ul>';
foreach ($result as $id) {
    $my_category = get_term_by('id', $id, 'my_category');
    $my_category_url = get_term_link($id, 'my_category');
    echo '<li><a href="' . $my_category_url . '">' . $my_category->name . '</a></li>';
}
echo '</ul>';