A unique index is similar to an RDBMS unique index, forbidding duplicate values for the indexed field. MongoDB creates a unique index by default on the _id field for every inserted document:

> db.books.createIndex( { "name": 1 }, { unique: true } )

This will create a unique index on a book's name.

A unique index can also be a compound embedded field or embedded document index.

In a compound index, the uniqueness is enforced across the combination of values in all fields of the index; for example, the following will not violate the unique index:

> db.books.createIndex( { "name": 1, "isbn": 1 }, { unique: true } )
> db.books.insert({"name": "Mastering MongoDB", "isbn": "101"})
> db.books.insert({"name": "Mastering MongoDB", "isbn": "102"})

This is because, even though the name is the same, our index is looking for the unique combination of {name,isbn} and the two entries differ on isbn.

Unique does not work with hashed indexes. Unique indexes cannot be created if the collection already contains duplicate values of the indexed field. A unique index will not prevent the same document from having multiple values.

If a document is missing the indexed field it will be inserted. If a second document is missing the indexed field, it will not be inserted. This is because MongoDB will store the missing field value as null, thus only allowing one document to be missing the field.

Indexes that are a combination of unique and partial will only apply unique after partial has been applied. This means that there may be several documents with duplicate values if they are not part of partial filtering.