public function testExplainLimit()
 {
     for ($i = 0; $i < 100; $i++) {
         $this->object->save(array("x" => $i));
     }
     $q = array("x" => array('$gt' => 50));
     $soft = $this->object->find($q)->limit(20)->explain();
     $hard = $this->object->find($q)->limit(-20)->explain();
     $this->assertEquals(20, $soft['n']);
     $this->assertEquals(20, $hard['n']);
 }