{

"_id"

: ObjectId(

"58b7ea9544e98b24a5bdcefd"

),

"i"

: 8,

"username"

:

"user8"

,

"age"

: 5,

"created"

:

"Thu Mar 02 2017 17:49:09 GMT+0800 (CS

T)"

}

{

"_id"

: ObjectId(

"58b7ea9544e98b24a5bdcefe"

),

"i"

: 9,

"username"

:

"user9"

,

"age"

: 56,

"created"

:

"Thu Mar 02 2017 17:49:09 GMT+0800 (C

ST)"

}

{

"_id"

: ObjectId(

"58b8da80d8509e8f46fd9042"

),

"i"

:

"10"

,

"age"

: 50

,

"create"

: ISODate(

"2017-03-03T02:52:48.834Z"

) }

这里可以看到，我在 helei 集合中生成了 i 从 0 到 10 这些数据，而 i:10 文档中我故意漏写

username 这一个键值

Part3:集合索引情况

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PRIMARY> db.helei.getIndexes()
[
{

"v"

: 1,

"key"

: {

"_id"

: 1

},

"name"

:

"_id_"

,

"ns"

:

"helei.helei"

},
{

"v"

: 1,

"key"

: {

"age"

: 1

},

"name"

:

"idx_age"

,

"ns"

:

"helei.helei"

},
{

"v"

: 1,

"unique"

:

true

,

"key"

: {

"username"

: 1

},

"name"

:

"uk_username"

,

"ns"

:

"helei.helei"

,

"background"

:

true

}
]