Using tax_query creates a 1 = 0 or 1 = 1 in $wp_query->request

This is my first attempt at modifying the main query using the pre_get_posts action/filter

This is the function currently hooked to it:

function advanced_product_search_filter($query)
{
  if(!is_admin() &&
     is_main_query() && 
     is_search() && $query->query_vars['post_type'] == 'product')
  {
    $keyword = $_GET['s'];

    if($_GET['exactly'])
      $keyword = $_GET['s'] . ' "'. $_GET['exactly'] . '"';

    if($_GET['without'])
    {
      $excluded = exclude_product_keyword_search($_GET['without'], 
                    $_GET['pname'],
                    $_GET['pcode']
                  );
      $query->set('post__not_in', $excluded);
    }

    if($_GET['pname'])
      $query->set('s', $keyword);
    else
      $query->set('s', '');

    if($_GET['pcode'])
    {
      $tax_queries = $query->get('tax_query');
      $tax_queries[] = array(
        array(
          'taxonomy' => 'pa_ordering-code',
          'field' => 'name',
          'terms' => array($keyword),
          'operator' => 'LIKE'
        )
      );
      $query->set('tax_query', $tax_queries);
    }
  }
}

This is a plugin built on top of a WooCommerce installation. So basically this function tries to support searching a WooCommerce product either through the standard search (title and content) or an attribute search on a custom attribute called Ordering Code which is pa_ordering-code in the term_taxonomy table

Other parts of the code works fine, I can modify the s and post__not_in vars without any problems, but if I try to use the tax_query it, the query seems to break down.

To give you an idea, when I try this:

?s=foo&post_type=product&pname=1&pcode=1&without=non&exactly=foo+bar

dumping the WHERE clause, I see this:

AND wp_posts.ID NOT IN (219) 
AND 0 = 1 
AND 
(
  (
    (wp_posts.post_title LIKE '%foo%') OR 
    (wp_posts.post_content LIKE '%foo%')
  ) AND (
    (wp_posts.post_title LIKE '%foo bar%') OR 
    (wp_posts.post_content LIKE '%foo bar%')
  )
) 
AND wp_posts.post_type="product" 
AND (wp_posts.post_status="publish" OR wp_posts.post_status="private") 
AND
( 
  (
    wp_postmeta.meta_key = '_visibility' AND
    CAST(wp_postmeta.meta_value AS CHAR) IN ('visible','search')
  ) 
)

Notice the AND 0 = 1? When I dump the JOIN clause, I become sure that the tax_query parameter isn’t being interpreted properly since I only see the postmeta table in there. No terms, term_relationships, or term_taxonomy table.

And I also tried simply overwriting the existing tax_query (if there is one) by doing this:

$args = array(
  array(
    'taxonomy' => 'pa_ordering-code',
    'field' => 'name',
    'terms' => array($keyword),
    'operator' => 'LIKE'
  )
);
$query->set('tax_query', $args);

But I still could not get it to work. Any ideas as to what I may be doing wrong?

The 0 = 1 happens when the terms don’t exist.

You’re creating a query for some set of terms. Before it can add the SQL to that main query, it first does a secondary query to get the term_id’s for the relevant terms. If none are found, then it returns 0 = 1 instead, to shortcut the main query because no terms match the request.