Objective c 嵌套objectForKey调用work，但等效值forkey调用不'；T

我有一本代表Flickr照片的词典

照片的描述如下所示：

{
    accuracy = 16;
    context = 0;
    dateupload = 1355106635;
    description =     {
        "_content" = "Barclays Center Arena\nAtlantic Yards\nProspect Heights\nBrooklyn, New York";
    };
    farm = 9;
    "geo_is_contact" = 0;
    "geo_is_family" = 0;
    "geo_is_friend" = 0;
    "geo_is_public" = 1;
    id = 8260097924;
    isfamily = 0;
    isfriend = 0;
    ispublic = 1;
    latitude = "40.681786";
    longitude = "-73.972545";
    originalformat = jpg;
    originalsecret = cd1bdd846a;
    owner = "62968578@N07";
    ownername = atlanticyardswebcam02;
    "place_id" = "uUNC_q9TWr1vROR4wA";
    secret = 320e308baa;
    server = 8502;
    tags = "newyork brooklyn construction prospectheights atlanticyards forestcityratner block1129 barclayscenterarena";
    title = "Barclays Center Arena - 20121209_2128";
    woeid = 28751452;
}

NSString *photoDescription = [[photo objectForKey:@"description"] objectForKey:@"_content"];

如果我通过嵌套的objectForKey调用获得描述，它就会工作。也就是说，像这样：

{
    accuracy = 16;
    context = 0;
    dateupload = 1355106635;
    description =     {
        "_content" = "Barclays Center Arena\nAtlantic Yards\nProspect Heights\nBrooklyn, New York";
    };
    farm = 9;
    "geo_is_contact" = 0;
    "geo_is_family" = 0;
    "geo_is_friend" = 0;
    "geo_is_public" = 1;
    id = 8260097924;
    isfamily = 0;
    isfriend = 0;
    ispublic = 1;
    latitude = "40.681786";
    longitude = "-73.972545";
    originalformat = jpg;
    originalsecret = cd1bdd846a;
    owner = "62968578@N07";
    ownername = atlanticyardswebcam02;
    "place_id" = "uUNC_q9TWr1vROR4wA";
    secret = 320e308baa;
    server = 8502;
    tags = "newyork brooklyn construction prospectheights atlanticyards forestcityratner block1129 barclayscenterarena";
    title = "Barclays Center Arena - 20121209_2128";
    woeid = 28751452;
}

NSString *photoDescription = [[photo objectForKey:@"description"] objectForKey:@"_content"];

这很笨拙，我的理解是这也应该有效：

NSString *photoDescription_kv = [photo valueForKey:@"description._content"];

但是，valueForKey始终返回null

---

我做错了什么？

应该改用valueForKeyPath

NSString *photoDescription_kv = [photo valueForKeyPath:@"description._content"];

---

应改为使用valueForKeyPath

NSString *photoDescription_kv = [photo valueForKeyPath:@"description._content"];