post meta
$meta_query = array( 'relation' => 'AND', array( 'key' => 'publisher', 'value' => 'test', ), array( 'key' => 'difficulty', 'value' => 'test', ), );
$meta_query = array( 'relation' => 'AND', array( 'key' => 'publisher', 'value' => 'test', ), array( 'key' => 'difficulty', 'value' => 'test', ), );