Wordpress trying to query two custom types to get child from the parent

Question

So basically I have these service categories, under that there are the actual services. I am basically trying to loop through the categories and display category title, then underneath that I want to loop through all of the corresponding child items. I'm using "Toolset" custom field plugin with Wordpress. I have the loop working for the "Service Categories" and now I am trying to loop through the "Service Categories" children custom post type of "Services".

Codewise this is what I have right now:

<?php
            $loop = new WP_Query(['post_type' => 'service-category']);
            while ( $loop->have_posts() ) : $loop->the_post();
            ?>

                <h2 class="title">
                    <?php echo the_title(); ?>
                    <?php if(types_render_field('subtitle')) {?>
                      <span class="subtitle"><?php echo types_render_field('subtitle');?></span>
                    <?php }?>
                </h2>


                <?php
                $loop2 = new WP_Query(['post_type' => 'service', 'post_parent' => get_the_ID()]);
                ?>

                <?php while ( $loop2->have_posts() ) : $loop2->the_post();?>

                <h2 class="title">
                    <?php echo the_title(); ?>
                    <?php if(types_render_field('subtitle')) {?>
                      <span class="subtitle"><?php echo types_render_field('subtitle');?></span>
                    <?php }?>
                </h2>

                <?php endwhile;?>


          <?php endwhile; ?>

This is a screenshot of how I am associating the child custom post "Service" to the parent "Services-Category".

$loop2 is where I am having the trouble trying to get the associated child items. How can I accomplish this? Any assistance would be greatly appreciated, I've burnt through two days searching for docs and all I find is the way to do it through the ui and not a template method.

@JacobPeattie When I look in the database to see the relationships I was able to find this:

So 36 is the parent Service Category post_id and 43 is the child Service post_id in the post_meta table. I was able to find the relationship I just have never done a meta_query()

Answer 1

So I was finally able to find what I needed:

                $loop2 = new WP_Query([
                  'post_type' => 'service',
                  'numberposts' => -1,
                  'meta_key' => '_wpcf_belongs_service-category_id',
                  'toolset_relationships' => [
                    'role' => 'child',
                    'related_to' => $parent_id,
                    'relationship' => ['service-category', 'service']
                  ]
                ]);

https://wp-types.com/documentation/customizing-sites-using-php/displaying-child-posts/

Answer 2

Based on what you've posted, we can see that the Service -> Service Category relationship is saved as post meta. So what you want to do is query posts that have the _wpcf_belongs_service-category_id meta value that matches the ID of the Service Category you're looking for.

So instead of querying post_parent, which is looking at the post_parent column of the posts table, not meta, you need to add a meta query. The documentation for that is here, but for your example you want to change

$loop2 = new WP_Query(['post_type' => 'service', 'post_parent' => get_the_ID()]);

To this:

$loop2 = new WP_Query([
    'post_type' => 'service', 
    'meta_key' => '_wpcf_belongs_service-category_id',
    'meta_value' => get_the_ID(),
]);