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1.2 upsert
upsert可以指定如果数据存在就更新，不存在就创建数据。
> db.users.find({"name":"yoona"});
>
> db.users.update({"name":"yoona"}, {name : "yoona", age:25, "school" : "xidian", type:1, favorites: {artist : "Noguchi", food : "nougat"}, finished : [4, 5] }, {upsert:false});
WriteResult({ "nMatched" : 0, "nUpserted" : 0, "nModified" : 0 })更新操作之前我们没有查询到yoona的任何信息，然后我们对其进行更新操作，并且upsert设置为false，表示如果更新的数据不存在，不走任何操作。更新之后，我们再次查询一下：
> db.users.find({"name":"yoona"});
>再次查询还是没有找到相应数据。我们设置upsert为true，表示如果数据存在则更新，如果不存在则创建该数据：
> db.users.update({"name":"yoona"}, {name : "yoona", age:25, "school" : "xidian", type:1, favorites: {artist : "Noguchi", food : "nougat"}, finished : [4, 5] }, {upsert:true});
WriteResult({
"nMatched" : 0,
"nUpserted" : 1,
"nModified" : 0,
"_id" : ObjectId("57c3ad26d2cc0133a95bc583")
})
>
> db.users.find({"name":"yoona"});
{ "_id" : ObjectId("57c3ad26d2cc0133a95bc583"), "name" : "yoona", "age" : 25, "school" : "xidian", "type" : 1, "favorites" : { "artist" : "Noguchi", "food" : "nougat" }, "finished" : [ 4, 5 ] }