WordPress query_posts with multiple tax_query

Question

The below query only works if $_POST['product_cat'] and $_POST['sub_category'] return something. How do I make this work if either one of these are empty?

In the end I will have 6 different optional taxonomies so I can't really have a separate query for every combination.

query_posts(array(
  'post_type' => 'product',
  'post_status' => 'publish',
  'tax_query' => array(
    'relation' => 'AND',
     array(
       'taxonomy' => 'product_cat',
       'terms' => $_POST['product_cat'],
     ),
     array(
       'taxonomy' => 'sub_category',
       // 'field' => 'slug',
       'terms' => $_POST['sub_category'],
     ),
     ...
   )
 ));

Answer 1

Easiest option is to construct the tax_query array based on the POST data and supply it to the WP_Query.

Example:

$tax_query = ['relation' => 'AND'];

// List the parameters you are getting from POST data
$post_filters = ['product_cat', 'sub_category', 'other_taxonomies'];

// Loop through the post parameters and add them to the tax_query
foreach ( $post_filters as $post_filter ) {
    if ( ! empty( $_POST[$post_filter] ) ) {
        $tax_query[] = [
            'taxonomy' => $post_filter,
            'terms' => $_POST[$post_filter]
        ];
    }
}

// And finally you can pass the $tax_query to the query
query_posts(array(
    'post_type' => 'product',
    'post_status' => 'publish',
    'tax_query' => $tax_query
));