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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 index, 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 the _id field itself for this kind of data rather than using a unique index on another field.

If a document does not have a value for a field, the index entry for that item will be null in any index that includes it. Thus, in many situations you will want to combine the unique constraint with the sparse option. Sparse indexes skip over any document that is missing the indexed field, rather than storing null for the index entry. 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

Deprecated since version 2.6: The dropDups option to ensureIndex(), createIndex(), and createIndexes is deprecated.

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.

Warning

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

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