Limit SearchWP results to chosen Category from dropdown.

searchwp-customizations.php

<?php

// Limit SearchWP results to chosen Category from dropdown.
// @link https://searchwp.com/documentation/knowledge-base/category-select-dropdown/
add_filter( 'searchwp\query\mods', function( $mods, $query ) {
	global $wpdb;

	// Only proceed if a Category was chosen from the dropdown.
	if ( ! isset( $_GET['swp_category_limiter'] ) || empty( intval( $_GET['swp_category_limiter'] ) ) ) {
		return $mods;
	}
	
	// Optional: only proceed if we're using a specific Engine.
	// if ( 'default' !== $query->get_engine()->get_name() ) {
	//	return $mods;
	// }

	$alias     = 'swpkbcat';
	$tax_query = new WP_Tax_Query( [ [
		'taxonomy' => 'category',
		'field'    => 'term_id',
		'terms'    => absint( $_GET['swp_category_limiter'] ),
	] ] );
	$tq_sql    = $tax_query->get_sql( $alias, 'ID' );
	$mod       = new \SearchWP\Mod();

	// If the JOIN is empty, WP_Tax_Query assumes we have a JOIN with wp_posts, so let's make that.
	if ( ! empty( $tq_sql['join'] ) ) {
		// Queue the assumed wp_posts JOIN using our alias.
		$mod->raw_join_sql( function( $runtime ) use ( $wpdb, $alias ) {
			return "LEFT JOIN {$wpdb->posts} {$alias} ON {$alias}.ID = {$runtime->get_foreign_alias()}.id";
		} );

		// Queue the WP_Tax_Query JOIN which already has our alias.
		$mod->raw_join_sql( $tq_sql['join'] );

		// Queue the WP_Tax_Query WHERE which already has our alias.
		$mod->raw_where_sql( '1=1 ' . $tq_sql['where'] );
	} else {
		// There's no JOIN here because WP_Tax_Query assumes a JOIN with wp_posts already
		// exists. We need to rebuild the tax_query SQL to use a functioning alias. The Mod
		// will ensure the JOIN, and we can use that Mod's alias to rebuild our tax_query.
		$mod->set_local_table( $wpdb->posts );
		$mod->on( 'ID', [ 'column' => 'id' ] );

		$mod->raw_where_sql( function( $runtime ) use ( $tax_query ) {
			$tq_sql = $tax_query->get_sql( $runtime->get_local_table_alias(), 'ID' );

			return '1=1 ' . $tq_sql['where'];
		} );
	}

	$mods[] = $mod;

	return $mods;
}, 20, 2 );