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"
}