syncIndex failed creating index for the field with name 'key'

[1nstinct]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.7.0

Node.js version

18.18.0

MongoDB server version

6.0.19

Typescript version (if applicable)

No response

Description

Schema

const ObjectKeySchema = new mongoose.Schema({
  key: {
    type: String,
    required: true,
    unique: true,
  },
  type: {
    type: String,
    required: false,
  },
}, {
  timestamps: false,
  versionKey: false,
});

ObjectKeySchema.index({ key: 1 });

The index key_1 already exists in the DB, but the function call ObjectKey.syncIndexes() throws an error:

MongoServerError: An existing index has the same name as the requested index. When index names are not specified, they are auto generated and can cause conflicts. Please refer to our documentation. Requested index: { v: 2, key: { key: 1 }, name: "key_1", background: true }, existing index: { v: 2, unique: true, key: { key: 1 }, name: "key_1", background: true }
at Connection.sendCommand (/data/node_modules/mongodb/lib/cmap/connection.js:289:27)
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
at async Connection.command (/data/node_modules/mongodb/lib/cmap/connection.js:312:26)
at async Server.command (/data/node_modules/mongodb/lib/sdam/server.js:167:29)
at async CreateIndexesOperation.executeCommand (/data/node_modules/mongodb/lib/operations/command.js:73:16)
at async CreateIndexesOperation.execute (/data/node_modules/mongodb/lib/operations/indexes.js:122:9)
at async tryOperation (/data/node_modules/mongodb/lib/operations/execute_operation.js:192:20)
at async executeOperation (/data/node_modules/mongodb/lib/operations/execute_operation.js:69:16)
at async Collection.createIndex (/data/node_modules/mongodb/lib/collection.js:326:25) {
  errorResponse: {
    ok: 0,
    errmsg: 'An existing index has the same name as the requested index. When index names are not specified, they are auto generated and can cause conflicts. Please refer to our documentation. Requested index: { v: 2, key: { key: 1 }, name: "key_1", background: true }, existing index: { v: 2, unique: true, key: { key: 1 }, name: "key_1", background: true }',
    code: 86,
    codeName: 'IndexKeySpecsConflict',
    '$clusterTime': {
      clusterTime: new Timestamp({ t: 1732303578, i: 14 }),
      signature: [Object]
    },
    operationTime: new Timestamp({ t: 1732303578, i: 14 })
  },
  ok: 0,
  code: 86,
  codeName: 'IndexKeySpecsConflict',
  '$clusterTime': {
    clusterTime: new Timestamp({ t: 1732303578, i: 14 }),
    signature: {
      hash: Binary.createFromBase64('OIA61Np4Zxmq4QBf1Ih5rquR7m0=', 0),
      keyId: new Long('7398342335731335169')
    }
  },
  operationTime: new Timestamp({ t: 1732303578, i: 14 }),
  [Symbol(errorLabels)]: Set(0) {}
}

Steps to Reproduce

1. Create schema
2. Run syncIndex()

Expected Behavior

Should ignore creating existing index.

[vkarpov15]

So the issue is that you're defining two separate indexes: one by specifying unique: true on the key schema path, and one with ObjectKeySchema.index({ key: 1 });. Not a Mongoose bug, the fix is to remove the ObjectKeySchema.index({ key: 1 }); call, which is unnecessary.

There are a couple of things Mongoose can do in this case, since the error message reported could be better.

1. Throw an error if there are multiple indexes with the exact same keys in a schema
2. Reconcile these index calls and make it so that ObjectKeySchema.index({ key: 1 }) is a no-op since there's already an index

I think (1) is the way to go, because (2) leads to unnecessary code that could easily be deleted, and would likely be more complex to implement. I'll put in a fix.