mongodb文档替换

对下面的文档做一个比较大的调整，将 friends、enemies两个字段移到 relationships子文档中。

> db.people.insert({ "name" : "joe", "friends" : 32, "enemies" : 2 });

> db.people.find();
{ "_id" : ObjectId("59afac49bc908a15fc4165b7"), "name" : "joe", "friends" : 32, "enemies" : 2 }

> db.users.insert({"name" : "joe","friends" : 32,"enemies" : 2})

> var joe=db.users.findOne({"name":"joe"});
> joe
{
    "_id" : ObjectId("59afad53825ebd8624a7ddd9"),
    "name" : "joe",
    "friends" : 32,
    "enemies" : 2
}

> joe.relationships = {"firends":joe.friends,"enemies":joe.enemies};
{ "firends" : 32, "enemies" : 2 }

> joe.username = joe.name;

> delete joe.friends;
 
> delete joe.enemies;
 
> delete joe.name;
 
> db.users.update({"name":"joe"},joe);

> db.users.findOne();
{
    "_id" : ObjectId("59afad53825ebd8624a7ddd9"),
    "relationships" : {
        "firends" : 32,
        "enemies" : 2
    },
    "username" : "joe"
}