Product variation WP_Query with a product category in Woocommerce

There is many mistakes in this WP_Query regarding Woocommerce products:

• For product category and product attribute you should better use normally a tax_query instead.
• For Product visibility, since Woocommerce 3, it's handled by product_visibility taxonomy for 'exclude-from-search' and 'exclude-from-catalog' terms.

Important notes about Product variations:

• Product categories (or product tags) are not handled by Product variations, but by the parent Variable product
• Product attributes for variations are handled as post meta data with a meta_key prepended by "attribute_" and a met_value that is the term slug.
• Product visibility is not handled in product variations, as they are not displayed in archives pages.
• They are not displayed in archive pages (as mentioned before).

So when using a WP_Query, you can NOT query at the same time "product" post type and "product_variation" post type as they are really different.

To make your query work for "product_variation" post type, you need a little utility function that will get the parent variable product for a product category (or any custom taxonomy as Product tags…):

// Utility function to get the parent variable product IDs for a any term of a taxonomy
function get_variation_parent_ids_from_term( $term, $taxonomy, $type ){
    global $wpdb;

    return $wpdb->get_col( "
        SELECT DISTINCT p.ID
        FROM {$wpdb->prefix}posts as p
        INNER JOIN {$wpdb->prefix}posts as p2 ON p2.post_parent = p.ID
        INNER JOIN {$wpdb->prefix}term_relationships as tr ON p.ID = tr.object_id
        INNER JOIN {$wpdb->prefix}term_taxonomy as tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
        INNER JOIN {$wpdb->prefix}terms as t ON tt.term_id = t.term_id
        WHERE p.post_type = 'product'
        AND p.post_status = 'publish'
        AND p2.post_status = 'publish'
        AND tt.taxonomy = '$taxonomy'
        AND t.$type = '$term'
    " );
}

Code goes in function.php file of your active child theme (or active theme). Tested and works. Necessary for the WP_Query below…

Here WP_Query code for Product variations (only) related to a specific product category and specific variation attribute values:

// Settings
$cat_name = 'Apple'; // Product category name
$attr_taxonomy = 'pa_size'; // Product attribute
$attribute_term_slugs = array('39'); // <== Need to be term SLUGs

$query = new WP_Query( array(
    'post_type'       => 'product_variation',
    'post_status'     => 'publish',
    'posts_per_page'  => 100,
    'post_parent__in' => get_variation_parent_ids_from_term( $cat_name, 'product_cat', 'name' ), // Variations
    'meta_query'      => array(
        'relation'    => 'AND',
        array(
            'key'     => '_stock',
            'value'   => 0,
            'compare' => '>'
        ),
        array( 
            'key'     => 'attribute_'.$attr_taxonomy, // Product variation attribute
            'value'   => $attribute_term_slugs, // Term slugs only
            'compare' => 'IN',
        ),
    ),
) );

// Display the queried products count
echo '<p>Product count: ' . $query->post_count . '<p>';

// Displaying raw output for posts
print_pr($query->posts);

Tested and works.