How do I taxonomy terms based on terms they are used alongside?

Question

How do I get all of a single taxonomies terms that have one or more particular sibling terms?

For example, let's say I have the following taxonomies. Terms are listed underneath the taxonomies, and the parenthesis indicates have many posts the terms are used on.

age taxonomy

• sixteen (used on 3 posts)
• twenty-seven (used on 0 posts)
• thirty-four (used on 12 posts)

eye_color taxonomy

• Blue (used on 5 posts)
• Brown (used on 10 posts)
• Green (used on 0 posts)

My desire is to get a list of all of the terms within eye_color that:

1. have posts
2. are used alongside thirty-four on one or more posts

Of course, I know that get_terms() has a hide_empty argument, but that only accomplishes half of what I want.

UPDATE: A comment below links to a function that allows you to find terms based on sibling terms of a single taxonomy...but I'd like to find terms based on sibling terms of multiple taxonomies.

In plain english, I'd basically like to:

Select all terms from taxonomy A that are used on a post with both Term 1 from Taxonomy B and Term 2 from Taxonomy C.

Any ideas?

Answer 1

So my solution on this kind of problems where we have a very complex set of rules is to avoid doing new SQL just for my solution because if you have to explain it to a beginner it gets hairy.

My solution:

function get_term_union( $taxonomy, $tax_query, $post_type = 'post' ){
    $args = array(
        'post_type' => $post_type,
        'tax_query' => $tax_query,
        'fields' => 'ids',
    );
    $posts = new WP_Query( $args );

    $terms = array();

    foreach( $posts as $post_id ){
        $terms = array_merge( $terms, wp_get_post_terms( $post_id, $taxonomy, array( 'fields' => 'ids' ) ) );
    }

    $terms = array_unique( $terms );

    return $terms;
}

// Usage
$tax_query = array(
    'relation' => 'AND',
    array(
        'taxonomy' => 'taxonomy_B',
        'field'    => 'slug',
        'terms'    => 'term_A',
    ),
    array(
        'taxonomy' => 'taxonomy_C',
        'field'    => 'slug',
        'terms'    => 'term_B',
    ),
);

// This will return you all the term id's for `taxonomy_A`
var_dump( get_term_union( 'taxonomy_A', $tax_query ) );

If this solution doesn't fit your needs tell me and I will try to fix it.

My Best Regards,