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Create a Unique Index

MongoDB allows you to specify a unique constraint on an index. These constraints prevent applications from inserting documents that have duplicate values for the inserted fields. Additionally, if you want to create an index on a collection that has existing data that might have duplicate values for the indexed field, you may choose to combine unique enforcement with duplicate dropping.

Unique Indexes

To create a unique indexes, consider the following prototype:

db.collection.ensureIndex( { a: 1 }, { unique: true } )

For example, you may want to create a unique index on the "tax-id": of the accounts collection to prevent storing multiple account records for the same legal entity:

db.accounts.ensureIndex( { "tax-id": 1 }, { unique: true } )

The _id index is a unique index. In some situations you may consider using _id field itself for this kind of data rather than using a unique index on another field.

In many situations you will want to combine the unique constraint with the sparse option. When MongoDB indexes a field, if a document does not have a value for a field, the index entry for that item will be null. Since unique indexes cannot have duplicate values for a field, without the sparse option, MongoDB will reject the second document and all subsequent documents without the indexed field. Consider the following prototype.

db.collection.ensureIndex( { a: 1 }, { unique: true, sparse: true } )

You can also enforce a unique constraint on compound indexes, as in the following prototype:

db.collection.ensureIndex( { a: 1, b: 1 }, { unique: true } )

These indexes enforce uniqueness for the combination of index keys and not for either key individually.

Drop Duplicates

To force the creation of a unique index index on a collection with duplicate values in the field you are indexing you can use the dropDups option. This will force MongoDB to create a unique index by deleting documents with duplicate values when building the index. Consider the following prototype invocation of ensureIndex():

db.collection.ensureIndex( { a: 1 }, { unique: true, dropDups: true } )

See the full documentation of duplicate dropping for more information.


Specifying { dropDups: true } may delete data from your database. Use with extreme caution.

Refer to the ensureIndex() documentation for additional index creation options.