遇到_id相同的情况下：

insert操作会报错；save完成保存操作

> db.person.find()> db.person.insert({"_id":1,age:1,name:"hh"})WriteResult({ "nInserted" : 1 })> db.person.find(){ "_id" : 1, "age" : 1, "name" : "hh" }#insert一个"_id"已经存在的文档> db.person.insert({"_id":1,age:10,name:"hh02"})WriteResult({        "nInserted" : 0,        "writeError" : {                "code" : 11000,                "errmsg" : "E11000 duplicate key error collection: foobar.person index: _id_ dup key: { : 1.0 }"        }})> db.person.find(){ "_id" : 1, "age" : 1, "name" : "hh" }#save一个"_id"已经存在的文档> db.person.save({"_id":1,age:10,name:"hh02"})WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })> db.person.find(){ "_id" : 1, "age" : 10, "name" : "hh02" }>