How to fix IndexAlreadyExists: Index with name already exists with a different key in MongoDB

Connect to your MongoDB instance and check what indexes currently exist on the collection:

// Using mongosh
db.yourCollection.getIndexes()

Look for the index name mentioned in the error message. Note its current key specification and options. Compare this with what your application is trying to create.

Once you've identified the conflicting index, drop it using its name:

// Using mongosh
db.yourCollection.dropIndex("index_name")

// Or using Node.js driver
await collection.dropIndex("index_name");

Warning: Dropping an index temporarily removes query optimization for queries using that index. Plan this during a maintenance window if the index is heavily used.

If you want to drop all indexes except _id (use with caution):

db.yourCollection.dropIndexes()

After dropping the old index, create the new index with your desired key specification:

// Using mongosh
db.yourCollection.createIndex(
  { fieldName: 1, anotherField: -1 },
  { name: "custom_index_name" }
)

// Or using Node.js driver
await collection.createIndex(
  { fieldName: 1, anotherField: -1 },
  { name: "custom_index_name" }
);

If you don't specify a name, MongoDB will auto-generate one based on the field names and sort order.

Alternatively, if you want to keep the old index and create a new one, simply use a different name:

// Create index with a unique, descriptive name
db.yourCollection.createIndex(
  { email: 1 },
  { name: "email_index_v2", unique: true }
)

This approach is useful during gradual migrations where you want both indexes available temporarily.

Ensure your application's index creation code matches your database state:

// Mongoose example - ensure index definitions are consistent
const userSchema = new Schema({
  email: { type: String, index: { unique: true, name: "email_unique_idx" } },
  username: String
});

// Or explicitly define indexes
userSchema.index({ email: 1 }, { unique: true, name: "email_unique_idx" });

Remove or update any old index definitions that reference the previous index name or specification.