Custom post taxonomies as tax_query terms?

Question

I have these two custom post types called “product” and “reference”. On “references” I have attached a custom taxonomy called “product_reference” which I use to specify which products the references are related to. On the “product” custom posts I have attached a custom module called “module-relevant-references” for fetching reference-posts relevant to the product.

This might sound a bit confusing as a whole so I have included a simplified picture to outline the setup.

What I need to do is to gather three relevant “reference” posts into the “relevant references” module (most likely with WP_Query’s tax_query filter parameter), but I haven’t been able to achieve this the intended way.

The filtering with WP_Query works if I fill in the tax_query terms manually, like in the code snippet below (product_category 1, 2 & 3):

$the_query = new WP_Query(array(
    'post_type'      => 'reference',
    'post_status'    => 'publish',
    'post__not_in'   => array( get_the_ID() ),
    'orderby'        => 'date',
    'order'          => 'DESC',
    'posts_per_page' => 3,
    'tax_query' => array(
        array(
            'taxonomy' => 'product_reference',
            'field'    => 'slug',
            'terms' => array(
                'product_category1',
                'product_category2',
                'product_category3'
            ),
            'hide_empty' => true,
            'include_children' => true,
            'operator' => 'IN'
        ),
    )
));

However I need this to work dynamically by getting the “product_reference” terms from all the relevant references as an array. The relevant-references module and the reference template are on different files.

I’ve tried using get_the_terms wp-function on the module php file, but it is unable to gather the information from the reference and returns “bool(false)”.

var_dump(get_the_terms(get_the_ID(), 'product_reference'));

wp_get_object_terms or wp_get_post_terms didn’t seem to do the trick either and both just returned array(0) { }.

So my question is: What is the correct way with this setup to fetch the relevant reference posts by filtering with “product_reference” terms?

I’m open for completely different approaches as well but at this point I’d really prefer not rebuilding the whole structure if possible.

Thanks for any help!

Answer 1

There is an example of how to do this on the official doc for WP_Query:

https://developer.wordpress.org/reference/classes/wp_query/#taxonomy-parameters

Specifically, by declaring multiple items in the tax_query and setting their relation:

Display posts that are in the quotes category OR have the quote post format:

$args = array(
    'post_type' => 'post',
    'tax_query' => array(
        'relation' => 'OR',
        array(
            'taxonomy' => 'category',
            'field'    => 'slug',
            'terms'    => array( 'quotes' ),
        ),
        array(
            'taxonomy' => 'post_format',
            'field'    => 'slug',
            'terms'    => array( 'post-format-quote' ),
        ),
    ),
);
$query = new WP_Query( $args );