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Sharded Collections with Unique Indexes

Unique indexes are not supported on sharded collections unless the index is the shard key or includes it as a prefix.

About this Task

The uniqueness constraint on an index ensures that documents in the collection have a unique value set on the field. With sharded collections, MongoDB does not enforce the uniqueness constraint on the field unless the index is the shard key or it includes the shard key as a prefix. To prevent issues:

On a sharded colelction, if you create a unique index that doesn't use the shard key, MongoDB returns an error to the createIndexes command.
If you shard a collection and the collection contains a unique index that doesn't use the shard key, MongoDB returns an error to the shardCollection command.

Steps

Create the shard key

Create the index you plan to use as the shard key:

db.names.createIndex( { region_id: 1 } )

region_id_1

Create a unique index

Create the unique index for the collection. Include the shard key as a prefix for the index:

db.names.createIndex(
   { region_id: 1, email: 1 },
   { unique: true }
)

region_id_1_email_1

Shard the collection

sh.shardCollection( "accounts.names", { region_id: 1 } )

{
   collectionsharded: 'accounts.names',
   ok: 1,
   '$clusterTime': {
      clusterTime: Timestamp( { t: 1759260515, i: 58 } ),
      signature: {
         hash: Binary.createFromBase64( 'AAAAAAAAAAAAAAAAAAAAAAAAAAA=', 0 ),
         keyId: Long( '0' )
      }
   },
   operationTime: Timestamp( { t: 1759260515, i: 57 } )
}

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