cruilla, comment to compare
$meta_query = array( 'relation' => 'AND', array( 'key' => 'comparar_con', 'compare' => 'EXISTS', ), array( 'key' => 'mgs-gprd-tested', 'value' => 'no', ), );
$meta_query = array( 'relation' => 'AND', array( 'key' => 'comparar_con', 'compare' => 'EXISTS', ), array( 'key' => 'mgs-gprd-tested', 'value' => 'no', ), );