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MongoDB _id Key的一些信息

关于 mongodb _id key:

_id key可以用户分配，也可以由mongodb自动分配，一般采用自动分配。
如果未使用_id作为分片key，则应用程序或客户端层要负责保证_id为唯一的，对于collection存在重复_id会有问题。If you do not use _id as the shard key, then your application/client layer must be responsible for keeping the _id field unique. It is problematic for collections to have duplicate _id values.
更新一个document 不会造成_ID被修改

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对于更新一个document 不会造成_ID被修改的证明:

> db.version(); 3.0.2  > db.dbdao_student.insert({student_name:"maclean",age:20,sex:"male",score:100}); WriteResult({ "nInserted" : 1 })  > db.dbdao_student.update({student_name:"maclean"},{"student_name" : "maclean", "age" : 20, "sex" : "male", "score" : 100}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > > db.dbdao_student.find(); { "_id" : ObjectId("5545bcff5714fbdfcb483510"), "student_name" : "maclean", "age" : 20, "sex" : "male", "score" : 100 } > > db.dbdao_student.update({student_name:"maclean"},{"student_name" : "maclean", "age" : 21, "sex" : "male", "score" : 100}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > db.dbdao_student.find(); { "_id" : ObjectId("5545bcff5714fbdfcb483510"), "student_name" : "maclean", "age" : 21, "sex" : "male", "score" : 100 } > > db.dbdao_student.update({student_name:"maclean"},{"student_name" : "maclean", "age" : 25, "sex" : "male", "score" : 100}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > > db.dbdao_student.find(); { "_id" : ObjectId("5545bcff5714fbdfcb483510"), "student_name" : "maclean", "age" : 25, "sex" : "male", "score" : 100 } > > db.dbdao_student.update({student_name:"maclean"},{"student_name" : "maclean", "age" : 25}); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 }) > db.dbdao_student.find(); { "_id" : ObjectId("5545bcff5714fbdfcb483510"), "student_name" : "maclean", "age" : 25 }  > db.dbdao_student.find(ObjectId("5545bcff5714fbdfcb483510")); { "_id" : ObjectId("5545bcff5714fbdfcb483510"), "student_name" : "maclean", "age" : 25 }

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