MongoDB – Unique Index

MongoDB allows us to specify a unique constraint on an index. These constraints  will prevent an  application from inserting documents that have duplicate values for the inserted fields.

Suppose we have  a collection named  things in a database stuff.


A unique index can be created by following command.

db.students.ensureIndex({student_id:1, class_id:1}, {unique:1});

The _id index is also a unique index. When we will try to insert same data in things collection MongoDB will throw a “duplicate key error”.

If we want to add a unique index on two fields then we can do it as follows. Suppose we have students collection and we want to add unique index on student_id and class id fields.

db.students.ensureIndex({student_id:1, class_id:1}, {unique:1});

Removing duplicates when creating unique indexes

We can remove duplicate key when creating an index, we can do this by adding dropDups: true option while adding a unique key index.

db.students.ensureIndex({student_id:1, class_id:1}, {unique:1, dropDups: true});

This option will remove all duplicates except one.

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