MongoDB -PHP data update

For save data from form to MongoDB database i'm using this code.

if (isset($_POST))
{   
    $m = new MongoClient();   
    $db = $m->abst;
    $collection = $db->users;
    print_r($_POST);    
    if($collection->find(array('user_id' => $pid)))
    {
        $collection->update(array('user_id'=>$pid),$_POST);
    }
    else
    { 
        $document = array_merge(array('user_id'=>$pid),$_POST);
        $collection->insert($document);   
    }  
}

How can check user id exist in collection? and if user id exist in collection, data will update. Otherwise new entry saved to collection. where is the mistake?