如何在 MongoDB 中设置主键？
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How to set a primary key in MongoDB?
提问by Tauquir
回答by Sinan
_idfield is reserved for primary key in mongodb, and that should be a unique value. If you don't set anything to _idit will automatically fill it with "MongoDB Id Object". But you can put any unique info into that field.
_idfield 是为 mongodb 中的主键保留的，它应该是一个唯一的值。如果你没有设置任何东西，_id它会自动用“MongoDB Id Object”填充它。但是您可以将任何唯一信息放入该字段。
Additional info: http://www.mongodb.org/display/DOCS/BSON
附加信息：http: //www.mongodb.org/display/DOCS/BSON
Hope it helps.
希望能帮助到你。
回答by amolgautam
回答by Ali
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.
在 mongodb 中 _id 字段是为主键保留的。如果您没有在对象中定义它并创建索引以确保性能，Mongodb 将使用内部 ObjectId 值。
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:
但是您可以为 _id 设置您自己的唯一值，Mongodb 将使用它而不是为您创建一个。即使您想使用多个字段作为主键，您也可以使用一个对象：
{ _id : { a : 1, b: 1} }
Just be careful when creating these ids that the order of keys (a and b in the example) matters, if you swap them around, it is considered a different object.
在创建这些 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对象。
回答by Nanhe Kumar
If you thinking like RDBMS you can't create primary key,default primary key is _id. But you can create Unique Index. Example is bellow.
如果你像 RDBMS 一样思考你不能创建主键，默认主键是 _id。但是您可以创建Unique Index。示例如下。
db.members.createIndex( { "user_id": 1 }, { unique: true } )
db.members.insert({'user_id':1,'name':'nanhe'})
db.members.insert({'name':'kumar'})
db.members.find();
Output is bellow.
输出如下。
{ "_id" : ObjectId("577f9cecd71d71fa1fb6f43a"), "user_id" : 1, "name" : "nanhe" }
{ "_id" : ObjectId("577f9cecd71d71fa1fb6f43a"), "user_id" : 1, "name" : "nanhe" }
{ "_id" : ObjectId("577f9d02d71d71fa1fb6f43b"), "name" : "kumar" }
{“_id”：ObjectId（“577f9d02d71d71fa1fb6f43b”），“名称”：“库马尔”}
When you try to insert same user_id mongodb trow write error.
当您尝试插入相同的 user_id mongodb trow 写入错误时。
db.members.insert({'user_id':1,'name':'aarush'})
WriteResult({ "nInserted" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate key error collection: student.members index: user_id_1 dup key: { : 1.0 }" } })
WriteResult({ "nInserted" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 重复键错误集合: student.members 索引: user_id_1 dup key: { : 1.0 }" } })
回答by Abhijit Chakra
Simple you can use db.collectionName.createIndex({urfield:1},{unique:true});
简单你可以使用 db.collectionName.createIndex({urfield:1},{unique:true});
回答by codiacTushki
One way of achieving this behaviour is by setting the value to _id(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 employee_idas the primary key then at the time of creating document in MongoDB; assign _idvalue same as that of employee_id.
实现此行为的一种方法是_id根据要视为主键的自定义字段将值设置为（为 MongoDB 中的主键保留）字段。
即如果我想employee_id作为主键，那么在 MongoDB 中创建文档时；赋值_id与 相同employee_id。
回答by nancy gupta
http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/
http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/
You can also check this link and auto-increment the _idfield.
您还可以检查此链接并自动增加该_id字段。
回答by Menuka Ishan
This is the syntax of creating primary key
这是创建主键的语法
db.< collection >.createIndex( < key and index type specification>, { unique: true } )
db.<集合>.createIndex(<键和索引类型说明>,{unique:true})
Let's take that our database have collection named studentand it's document have key named student_idwhich we need to make a primary key. Then the command should be like below.
让我们假设我们的数据库有一个名为student 的集合，它的文档有一个名为student_id 的键，我们需要它来做一个主键。然后命令应该如下所示。
db.student.createIndex({student_id:1},{unique:true})
You can check whether this student_idset as primary key by trying to add duplicate value to the studentcollection.
您可以通过尝试向学生集合添加重复值来检查此student_id是否设置为主键。
prefer this document for further informations https://docs.mongodb.com/manual/core/index-unique/#create-a-unique-index
更喜欢本文档以获取更多信息 https://docs.mongodb.com/manual/core/index-unique/#create-a-unique-index
回答by Tincho825
If you're using Mongo on Meteor, you can use _ensureIndex:
如果你在 Meteor 上使用 Mongo，你可以使用_ensureIndex：
CollectionName._ensureIndex({field:1 }, {unique: true});
