为片式集合强制使用唯一密钥

Process¶

To shard a collection using the unique constraint, specify the shardCollection command in the following form:

db.runCommand( { shardCollection : "test.users" , key : { email : 1 } , unique : true } );

Remember that the _id field index is always unique. By default, MongoDB inserts an ObjectId into the _id field. However, you can manually insert your own value into the _id field and use this as the shard key. To use the _id field as the shard key, use the following operation:

db.runCommand( { shardCollection : "test.users" } )

Limitations¶

• You can only enforce uniqueness on one single field in the collection using this method.
• If you use a compound shard key, you can only enforce uniqueness on the combination of component keys in the shard key.

In most cases, the best shard keys are compound keys that include elements that permit write scaling and query isolation, as well as high cardinality. These ideal shard keys are not often the same keys that require uniqueness and requires a different approach.

Process¶

Consider the following for the “proxy collection:”

{
  "_id" : ObjectId("...")
  "email" ": "..."
}

The _id field holds the ObjectId of the document it reflects, and the email field is the field on which you want to ensure uniqueness.

To shard this collection, use the following operation using the email field as the shard key:

db.runCommand( { shardCollection : "records.proxy" , key : { email : 1 } , unique : true } );

If you do not need to shard the proxy collection, use the following command to create a unique index on the email field:

db.proxy.ensureIndex( { "email" : 1 }, {unique : true} )

You may create multiple unique indexes on this collection if you do not plan to shard the proxy collection.

To insert documents, use the following procedure in the JavaScript shell:

use records;

var primary_id = ObjectId();

db.proxy.insert({
   "_id" : primary_id
   "email" : "example@example.net"
})

// if: the above operation returns successfully,
// then continue:

db.information.insert({
   "_id" : primary_id
   "email": "example@example.net"
   // additional information...
})

You must insert a document into the proxy collection first. If this operation succeeds, the email field is unique, and you may continue by inserting the actual document into the information collection.

Considerations¶

• Your application must catch errors when inserting documents into the “proxy” collection and must enforce consistency between the two collections.
• If the proxy collection requires sharding, you must shard on the single field on which you want to enforce uniqueness.
• To enforce uniqueness on more than one field using sharded proxy collections, you must have one proxy collection for every field for which to enforce uniqueness. If you create multiple unique indexes on a single proxy collection, you will not be able to shard proxy collections.