blocks|key|302704|text|_id字段是为mongodb中的主键保留的，应该是唯一的值。如果你没有设置任何对象，它会自动填充"MongoDB+Id+_id“。但您可以在该字段中输入任何独特的信息。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|302705|更多信息：http://www.mongodb.org/display/DOCS/BSON|302706|希望能有所帮助。|302707|entityMap|0|LINK|mutability|MUTABLE|url|http://www.mongodb.org/display/DOCS/BSON^0|0|3|1O|3|0|5|14|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|H|3|I|5|6|7|10|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]]]

<code>_id</code> field is reserved for primary key in mongodb, and that should be a unique value. If you don't set anything to <code>_id</code> it will automatically fill it with "MongoDB Id Object". But you can put any unique info into that field.

Additional info: <a href="http://www.mongodb.org/display/DOCS/BSON" rel="noreferrer">http://www.mongodb.org/display/DOCS/BSON</a>

Hope it helps.

blocks|key|3032648|text|另一种方法是为您的集合创建Indexes，并确保它们是唯一的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3032649|您可以在以下link上找到更多信息|3032650|实际上，我发现这很简单，很容易实现。|3032651|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.org/master/tutorial/create-a-unique-index/^0|D|7|0|6|4|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@]|D|@$9|V|A|W|1|X]]|E|$]]|$1|H|3|I|5|6|7|Y|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|K|$L|$5|M|N|O|E|$P|Q]]]]

The other way is to create <code>Indexes</code> for your collection and make sure that they are unique.

You can find more on the following <a href="https://docs.mongodb.org/master/tutorial/create-a-unique-index/">link</a>

I actually find this pretty simple and easy to implement.

blocks|key|304925|text|在mongodb中，_id字段保留为主键。如果您没有在您的对象中定义，那么Mongodb使用一个内部的ObjectId值，并创建一个索引以确保性能。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|304926|但是您可以为_id设置自己的唯一值，Mongodb将使用它，而不是为您创建一个。即使你想使用多个字段作为主键，你也可以使用一个对象：|304927|{+_id+:+{+a+:+1,+b:+1}+}|code-block|syntax|javascript|304928|当使用object作为id时要小心，键的顺序(示例中的a和b)很重要，如果你交换它们，它被认为是一个不同的id。|304929|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

In mongodb _id field is reserved for primary key. Mongodb use an internal ObjectId value if you don't define it in your object and also create an index to ensure performance.
But you can put your own unique value for _id and Mongodb will use it instead of making one for you. And even if you want to use multiple field as primary key you can use an object:
<pre><code>{ _id : { a : 1, b: 1} }
</code></pre>
Just be careful when using object as ids, the order of keys (a and b in the example) matters, if you swap them, it is considered a different id.

blocks|key|3032773|text|如果你像RDBMS一样思考，你不能创建主键。默认主键为_id。但是你可以创建Unique+Index。下面是一个例子。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3032774|db.members.createIndex(+{+"user_id":+1+},+{+unique:+true+}+)

db.members.insert({'user_id':1,'name':'nanhe'})

db.members.insert({'name':'kumar'})

db.members.find();|code-block|syntax|javascript|3032775|输出如下：|3032776|{+"_id“：ObjectId("577f9cecd71d71fa1fb6f43a")，"user_id”：1，"name“：”南河“}|3032777|{+"_id“：ObjectId("577f9d02d71d71fa1fb6f43b")，"name”："kumar“}|3032778|当你尝试插入相同的user_id时，mongodb会抛出一个写错误。|3032779|db.members.insert({'user_id':1,'name':'aarush'})|3032780|WriteResult({+"nInserted“：11000，"writeError”：{+"code“：11000，"errmsg”："E11000重复密钥错误集合:+student.members索引:+user_id_1重复密钥：{：1.0+}“}+})|3032781|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.com/manual/core/index-unique/^0|12|C|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]]|C|$]]|$1|D|3|E|5|F|7|16|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|17|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|18|8|@]|9|@]|C|$]]|$1|M|3|N|5|6|7|19|8|@]|9|@]|C|$]]|$1|O|3|P|5|6|7|1A|8|@]|9|@]|C|$]]|$1|Q|3|R|5|F|7|1B|8|@]|9|@]|C|$G|H]]|$1|S|3|T|5|6|7|1C|8|@]|9|@]|C|$]]|$1|U|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|V|$W|$5|X|Y|Z|C|$10|11]]]]

If you thinking like RDBMS, you can't create primary key. Default primary key is _id.
But you can create <a href="https://docs.mongodb.com/manual/core/index-unique/" rel="nofollow noreferrer">Unique Index</a>. Example is bellow.
<pre><code>db.members.createIndex( { &quot;user_id&quot;: 1 }, { unique: true } )

db.members.insert({'user_id':1,'name':'nanhe'})

db.members.insert({'name':'kumar'})

db.members.find();
</code></pre>
Output is bellow :
{ &quot;_id&quot; : ObjectId(&quot;577f9cecd71d71fa1fb6f43a&quot;), &quot;user_id&quot; : 1, &quot;name&quot; : &quot;nanhe&quot; }
{ &quot;_id&quot; : ObjectId(&quot;577f9d02d71d71fa1fb6f43b&quot;), &quot;name&quot; : &quot;kumar&quot; }
When you try to insert same user_id mongodb throws a write error.
<pre><code>db.members.insert({'user_id':1,'name':'aarush'})
</code></pre>
WriteResult({
&quot;nInserted&quot; : 0,
&quot;writeError&quot; : {
&quot;code&quot; : 11000,
&quot;errmsg&quot; : &quot;E11000 duplicate key error collection: student.members index: user_id_1 dup key: { : 1.0 }&quot;
}
})

blocks|key|302163|text|很简单，你可以使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|302164|db.collectionName.createIndex({urfield:1},{unique:true});|code-block|syntax|javascript|302165|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Simple you can use
<pre><code>db.collectionName.createIndex({urfield:1},{unique:true});
</code></pre>

blocks|key|302737|text|实现此行为的一种方法是，根据要作为主键处理的自定义字段，将值设置为_id+(在MongoDB中为主键保留)字段。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|302738|例如，如果我想要employee_id作为主键，那么在MongoDB中创建文档时；分配与employee_id相同的_id值。|302739|entityMap^0|X|3|0|8|B|18|B|1M|3|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

One way of achieving this behaviour is by setting the value to <code>_id</code> (which is reserved for a primary key in MongoDB) field based on the custom fields you want to treat as primary key. 
i.e. If I want <code>employee_id</code> as the primary key then at the time of creating document in MongoDB; assign <code>_id</code> value same as that of <code>employee_id</code>.

blocks|key|302916|text|这是创建主键的语法|type|unstyled|depth|inlineStyleRanges|entityRanges|data|302917|唯一db.<集合>.createIndex(+<键和索引类型specification>，{+unique:+true+})|302918|302919|让我们假设我们数据库有一个名为student的集合，它的文档有一个名为student_id的键，我们需要这个键来创建一个主键。那么命令应该如下所示。|offset|length|style|BOLD|302920|db.student.createIndex({student_id:1},{unique:true})|code-block|syntax|javascript|302921|您可以通过尝试向student_id集合添加重复值来检查此是否设置为主键。|302922|有关详细信息，请参阅此文档https://docs.mongodb.com/manual/core/index-unique/#create-a-unique-index|302923|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.com/manual/core/index-unique/#create-a-unique-index^0|0|0|0|F|7|Z|A|0|0|8|A|0|D|20|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|12|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|13|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|14|8|@$G|15|H|16|I|J]|$G|17|H|18|I|J]]|9|@]|A|$]]|$1|K|3|L|5|M|7|19|8|@]|9|@]|A|$N|O]]|$1|P|3|Q|5|6|7|1A|8|@$G|1B|H|1C|I|J]]|9|@]|A|$]]|$1|R|3|S|5|6|7|1D|8|@]|9|@$G|1E|H|1F|1|1G]]|A|$]]|$1|T|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]]|U|$V|$5|W|X|Y|A|$Z|10]]]]

This is the syntax of creating primary key 

<blockquote>
 db.&lt; collection >.createIndex( &lt; key and index type specification>, {
 unique: true } )
</blockquote>

Let's take that our database have collection named student and it's document have key named student_id which we need to make a primary key. Then the command should be like below.

<pre><code>db.student.createIndex({student_id:1},{unique:true})
</code></pre>

You can check whether this student_id set as primary key by trying to add duplicate value to the student collection.

prefer this document for further informations <a href="https://docs.mongodb.com/manual/core/index-unique/#create-a-unique-index" rel="nofollow noreferrer">https://docs.mongodb.com/manual/core/index-unique/#create-a-unique-index</a>

blocks|key|299712|text|MongoDB+>+Tutorial+>+Create+an+auto+incrementing+field+-+Archive|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|299713|您还可以选中此链接并自动递增_id字段。|style|CODE|299714|entityMap|0|LINK|mutability|MUTABLE|url|https://web.archive.org/web/20130803163812/http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/^0|0|1S|0|0|E|3|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@$A|U|B|V|F|G]]|9|@]|C|$]]|$1|H|3|-4|5|6|7|W|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]]]

<a href="https://web.archive.org/web/20130803163812/http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/" rel="nofollow noreferrer">MongoDB &gt; Tutorial &gt; Create an auto incrementing field - Archive</a>
You can also check this link and auto-increment the <code>_id</code> field.

blocks|key|3032861|text|如果你使用的是Mongo+on+Meteor，你可以使用_ensureIndex|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3032862|CollectionName._ensureIndex({field:1+},+{unique:+true});|code-block|syntax|javascript|3032863|entityMap^0|S|C|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

If you're using Mongo on Meteor, you can use <code>_ensureIndex</code>:

<pre><code>CollectionName._ensureIndex({field:1 }, {unique: true});
</code></pre>

I want to set one of my fields as primary key. I am using <a href="http://www.mongodb.org/" rel="noreferrer">MongoDB</a> as my NoSQL.

问如何在MongoDB中设置主键？
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