MongoDB unique index

link

PRIMARY> db.helei.find()　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcef5"), "i" : 0, "username" : "user0", "age" : 8, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcef6"), "i" : 1, "username" : "user1", "age" : 9, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcef7"), "i" : 2, "username" : "user2", "age" : 82, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcef8"), "i" : 3, "username" : "user3", "age" : 48, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcef9"), "i" : 4, "username" : "user4", "age" : 27, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcefa"), "i" : 5, "username" : "user5", "age" : 53, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcefb"), "i" : 6, "username" : "user6", "age" : 42, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcefc"), "i" : 7, "username" : "user7", "age" : 56, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcefd"), "i" : 8, "username" : "user8", "age" : 5, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b7ea9544e98b24a5bdcefe"), "i" : 9, "username" : "user9", "age" : 56, "created" : "Thu Mar 02 2017 17:49:09 GMT+0800 (CST)" }
　　
{ "_id" : ObjectId("58b8da80d8509e8f46fd9042"), "i" : "10", "age" : 50, "create" : ISODate("2017-03-03T02:52:48.834Z") }