MongoDB内嵌文档的索引无效：报错

正在学习MongoDB到“内嵌索引”部分，动手过程如下：

peoples = [
{"name":{"first":"joe", "last":"schome"}, "age":27},
{"name":{"first":"simon", "last":"wade"}, "age":18},
{"name":{"first":"joe", "last":"kelly"}, "age":30}
]

成功插入集合：

> db.peoples.insert(peoples)
BulkWriteResult({
        "writeErrors" : [ ],
        "writeConcernErrors" : [ ],
        "nInserted" : 3,
        "nUpserted" : 0,
        "nMatched" : 0,
        "nModified" : 0,
        "nRemoved" : 0,
        "upserted" : [ ]
})

> db.peoples.ensureIndex({"name.first":1, "age":-1})
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}
> db.peoples.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "test.peoples"
        },
        {
                "v" : 1,
                "key" : {
                        "name.first" : 1,
                        "age" : -1
                },
                "name" : "name.first_1_age_-1",
                "ns" : "test.peoples"
        }
]

> db.peoples.find().pretty()
{
        "_id" : ObjectId("556dcd540d56f64140620b54"),
        "name" : {
                "first" : "joe",
                "last" : "schome"
        },
        "age" : 27
}
{
        "_id" : ObjectId("556dcd540d56f64140620b55"),
        "name" : {
                "first" : "simon",
                "last" : "wade"
        },
        "age" : 18
}
{
        "_id" : ObjectId("556dcd540d56f64140620b56"),
        "name" : {
                "first" : "joe",
                "last" : "kelly"
        },
        "age" : 30
}
>

于是，尝试删除上面创建的索引，对age创建一个简单的索引：

> db.peoples.dropIndex("name.first_1_age_-1")
{ "nIndexesWas" : 2, "ok" : 1 }
> db.peoples.ensureIndex({"age":-1})
{
        "createdCollectionAutomatically" : false,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}
> db.peoples.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "test.peoples"
        },
        {
                "v" : 1,
                "key" : {
                        "age" : -1
                },
                "name" : "age_-1",
                "ns" : "test.peoples"
        }
]

> db.peoples.find().pretty()
{
        "_id" : ObjectId("556dcd540d56f64140620b54"),
        "name" : {
                "first" : "joe",
                "last" : "schome"
        },
        "age" : 27
}
{
        "_id" : ObjectId("556dcd540d56f64140620b55"),
        "name" : {
                "first" : "simon",
                "last" : "wade"
        },
        "age" : 18
}
{
        "_id" : ObjectId("556dcd540d56f64140620b56"),
        "name" : {
                "first" : "joe",
                "last" : "kelly"
        },
        "age" : 30
}

真是百思不得其解~