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Unique Indexes

A unique index ensures that the indexed fields do not store duplicate values; i.e. enforces uniqueness for the indexed fields. By default, MongoDB creates a unique index on the _id field during the creation of a collection.

Create a Unique Index

To create a unique index, use the db.collection.createIndex() method with the unique option set to true.

db.collection.createIndex( <key and index type specification>, { unique: true } )

Unique Index on a Single Field

For example, to create a unique index on the user_id field of the members collection, use the following operation in the mongo shell:

db.members.createIndex( { "user_id": 1 }, { unique: true } )

Unique Compound Index

You can also enforce a unique constraint on compound indexes. If you use the unique constraint on a compound index, then MongoDB will enforce uniqueness on the combination of the index key values.

For example, to create a unique index on groupNumber, lastname, and firstname fields of the members collection, use the following operation in the mongo shell:

db.members.createIndex( { groupNumber: 1, lastname: 1, firstname: 1 }, { unique: true } )

The created index enforces uniqueness for the combination of groupNumber, lastname, and firstname values.

Behavior

Restrictions

MongoDB cannot create a unique index on the specified index field(s) if the collection already contains data that would violate the unique constraint for the index.

You may not specify a unique constraint on a hashed index.

Unique Constraint Across Separate Documents

The unique constraint applies to separate documents in the collection. That is, the unique index prevents separate documents from having the same value for the indexed key, but the index does not prevent a document from having multiple elements or embedded documents in an indexed array from having the same value. In the case of a single document with repeating values, the repeated value is inserted into the index only once.

For example, a collection has a unique index on a.b:

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

The unique index permits the insertion of the following document into the collection if no other document in the collection has the a.b value of 5:

db.collection.insert( { a: [ { b: 5 }, { b: 5 } ] } )

Unique Index and Missing Field

If a document does not have a value for the indexed field in a unique index, the index will store a null value for this document. Because of the unique constraint, MongoDB will only permit one document that lacks the indexed field. If there is more than one document without a value for the indexed field or is missing the indexed field, the index build will fail with a duplicate key error.

For example, a collection has a unique index on x:

db.collection.createIndex( { "x": 1 }, { unique: true } )

The unique index allows the insertion of a document without the field x if the collection does not already contain a document missing the field x:

db.collection.insert( { y: 1 } )

However, the unique index errors on the insertion of a document without the field x if the collection already contains a document missing the field x:

db.collection.insert( { z: 1 } )

The operation fails to insert the document because of the violation of the unique constraint on the value of the field x:

WriteResult({
   "nInserted" : 0,
   "writeError" : {
      "code" : 11000,
      "errmsg" : "E11000 duplicate key error index: test.collection.$a.b_1 dup key: { : null }"
   }
})

Unique Partial Indexes

New in version 3.2.

Partial indexes only index the documents in a collection that meet a specified filter expression. If you specify both the partialFilterExpression and a unique constraint, the unique constraint only applies to the documents that meet the filter expression.

A partial index with a unique constraint does not prevent the insertion of documents that do not meet the unique constraint if the documents do not meet the filter criteria. For an example, see Partial Index with Unique Constraint.