How to put posts with some taxonomy on top of others in `pre_get_posts`

Question

The following tax_query returns only matched posts (with 'matchedstring' IN taxonomy array):

function only_returns_matched_posts( $query ) {

    if( !$query->is_main_query() || is_admin() )
        return;
    
    $taxquery = array(
        array(
            'taxonomy' => 'mygroup',
            'field' => 'slug',
            'terms' => 'matchedstring',
            'compare'=> 'IN'
        )
    );
    $query->set( 'tax_query', $taxquery );
    
}
add_action( 'pre_get_posts', 'only_returns_matched_posts' );

I want the matched posts grouped at the top of the query with the other posts following. Is it possible to either:

• use this format with a orderby
• do 2 separate queries and merge them
• use a custom Group By SQL query

EDIT

I managed to merge 2 queries but I lose the menu_order when I apply post__in to keep the $queryA + $queryB order. Should I get ids differently than with $query->posts to keep original menu_order of the queries?

function group_matched_posts_at_top( $query ) {

  // Check if this is the main query and not in the admin area
  if( !$query->is_main_query() || is_admin() )
    return;

  // Get posts with matched taxonomy + posts without taxonomy
  $queryAparams = array(
    'posts_per_page' => -1,
    'post_type' => 'product',
    'order_by' => 'menu_order',
    'order' => 'ASC',
    'fields' => 'ids',
    'tax_query'=> array(
      'relation' => 'OR',
      array(
        'taxonomy' => 'pa_group',
        'field' => 'slug',
        'terms' => 'matchedstring',
        'operator' => 'IN'
      ),
      array(
        'taxonomy' => 'pa_group',
        'operator' => 'NOT EXISTS'
      )
    )
  );

  // Get posts with other taxonomies
  $queryBparams = array(
    'posts_per_page' => -1,
    'post_type' => 'product',
    'order_by' => 'menu_order',
    'order' => 'ASC',
    'fields' => 'ids',
    'tax_query'=> array(
      'relation' => 'AND',
      array(
        'taxonomy' => 'pa_group',
        'field' => 'slug',
        'terms' => 'matchedstring',
        'operator' => 'NOT IN'
      ),
      array(
        'taxonomy' => 'pa_group',
        'operator' => 'EXISTS'
      )
    )
  );

  $queryA = new WP_Query($queryAparams);
  $queryB = new WP_Query($queryBparams);
  
  // Merging ids
  $postIDs = array_merge($queryA->posts,$queryB->posts);

  if(!empty($postIDs)){
      $query->set('post__in', $postIDs);
      $query->set('orderby', 'post__in'); 
  }

}
add_action( 'woocommerce_product_query', 'group_matched_posts_at_top' );

EDIT2

I'll post my own answer. I had to actually remove the 'fields' => 'ids' parameters to keep the queries menu_order and pluck the ids after resorting them.

Answer 1

You could try modifying the tax query to use the relation parameter and add a second clause that matches any post that does not have the matched string value in the meta array.

See Taxonomy Parameters.

EDIT: Thank you for pointing that out, Tom. You're correct, I've updated to reflect.

function group_matched_posts_at_top( $query ) {

// Check if this is the main query and not in the admin area
if( !$query->is_main_query() || is_admin() )
    return;

// Define the tax query with two clauses (matched and not matched)
$taxquery = array(
    'relation' => 'OR', // Set the relation to OR to include posts that match either clause
    array(
        'taxonomy' => 'mymeta',
        'field' => 'slug',
        'terms' => 'matchedstring',
        'operator' => 'IN' // use the operator parameter to specify the comparison operator
    ),
    array(
        'taxonomy' => 'mymeta',
        'field' => 'slug',
        'terms' => 'matchedstring',
        'operator' => 'NOT IN' // use the operator parameter to specify the comparison operator
    )
);

// Set the tax query and meta key parameters for the query
$query->set( 'tax_query', $taxquery );
$query->set( 'meta_key', 'mymeta' );

// Set the orderby parameter to sort by the value of the "mymeta" field in descending order (so that matched posts appear first), and then by date in descending order (so that the most recent posts appear first within each group).
$query->set( 'orderby', array( 'meta_value' => 'DESC', 'date' => 'DESC' ) );


add_action( 'pre_get_posts', 'group_matched_posts_at_top' );

The main changes are as follows:

• I've used the "taxonomy" parameter to specify the taxonomy to query.

• I've used the "operator" parameter instead of "compare" to specify the comparison operator (IN or NOT IN).

• I've added the "field" parameter with a value of "slug" to specify that we're comparing the term slug (i.e., the term's "mymeta" field).

These changes should make the query work with a term/taxonomy meta field, which was not supported by my earlier solution.

Answer 2

I finally managed to do it by merging 2 queries. I'm using it with a woocommerce attribute but it can work with any taxonomy.

function group_matched_posts_at_top( $query ) {

    // Modify the query only if it's the main query and it's a product archive page
    if ( !$query->is_main_query() || !(is_product_category() || is_shop())  || is_admin() ) {
        return;
    }

  // String to match
  $string_to_match = 'Whatever taxonomy';

  // Query for products with matched taxonomy
  $query1 = new WP_Query( array(
    'post_type' => 'product',
    'posts_per_page' => -1,
    'post_status' => 'publish',
    'order_by' => 'menu_order',
    'order' => 'ASC',
      'tax_query'=> array(
      'relation' => 'OR',
      array(
        'taxonomy' => 'pa_group',
        'field' => 'name',
        'terms' => $string_to_match,
        'operator' => 'IN'
      ),
      array(
        'taxonomy' => 'pa_group',
        'operator' => 'NOT EXISTS'
      )
    )
  ) );
  
  // Sorting first query before merging
  usort( $query1->posts, function( $a, $b ) {
    return $a->menu_order - $b->menu_order;
  } );

  // Query for other products
  $query2 = new WP_Query( array(
    'post_type' => 'product',
    'posts_per_page' => -1,
    'post_status' => 'publish',
    'order_by' => 'menu_order',
    'order' => 'ASC',
      'tax_query'=> array(
      'relation' => 'AND',
      array(
        'taxonomy' => 'pa_group',
        'field' => 'name',
        'terms' => $string_to_match,
        'operator' => 'NOT IN'
      ),
      array(
        'taxonomy' => 'pa_group',
        'operator' => 'EXISTS'
      )
    )
  ) );

  // Sorting second query before merging
  usort( $query2->posts, function( $a, $b ) {
    return $a->menu_order - $b->menu_order;
  } );

  // Merge the results in the desired order
  $products = array_merge( $query1->posts, $query2->posts );

  // Set the modified query results
  $query->set( 'posts_per_page', -1 );
  $query->set( 'post__in', wp_list_pluck( $products, 'ID' ) );
  // keep the order
  $query->set( 'orderby', 'post__in' );
}

add_action( 'pre_get_posts', 'group_matched_posts_at_top' );