Convert an Existing Index to a Unique Index

Populate sample data

Create the apples collection:

db.apples.insertMany( [
   { type: "Delicious", quantity: 12 },
   { type: "Macintosh", quantity: 13 },
   { type: "Delicious", quantity: 13 },
   { type: "Fuji", quantity: 15 },
   { type: "Washington", quantity: 10 }
] )

Create a single field index

Add a single field index on the type field:

db.apples.createIndex( { type: 1 } )

Prepare the index to be converted to a unique index

Run collMod on the type field index and set prepareUnique to true:

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      prepareUnique: true
   }
} )

After prepareUnique is set, you cannot insert new documents that duplicate an index key entry. For example, the following insert operation results in an error:

db.apples.insertOne( { type: "Delicious", quantity: 20 } )

MongoServerError: E11000 duplicate key error collection:
test.apples index: type_1 dup key: { type: "Delicious" }

Check for unique key violations

To see if there are any documents that violate the unique constraint on the type field, run collMod with unique: true and dryRun: true:

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      unique: true
   },
   dryRun: true
} )

MongoServerError: Cannot convert the index to unique. Please resolve conflicting documents before running collMod again.
Violations: [
   {
      ids: [
         ObjectId("660489d24cabd75abebadbd0"),
         ObjectId("660489d24cabd75abebadbd2")
      ]
   }
]

Resolve duplicate key conflicts

To complete the conversion, modify the duplicate entries to remove any conflicts. For example:

db.apples.deleteOne(
   { _id: ObjectId("660489d24cabd75abebadbd2") }
)

Confirm that all conflicts are resolved

To confirm that the index can be converted, re-run the collMod() command with dryRun: true:

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      unique: true
   },
   dryRun: true
} )

{ ok: 1 }

Finalize the index conversion

To finalize the conversion to a unique index, run the collMod command with unique: true and remove the dryRun flag:

db.runCommand( {
   collMod: "apples",
   index: {
      keyPattern: { type: 1 },
      unique: true
   }
} )

{ unique_new: true, ok: 1 }