index setting error occurred at 8.9.1("Schema already has an index")

[0Ams]

Prerequisites

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Issue

Error message

MongooseError: Schema already has an index on {"uid":1,"gid":1}
    at Schema.index (xxx/node_modules/mongoose/lib/schema.js:2151:13)
    at Object.<anonymous> (xx/schema/member.js:20:8)

Schema

/* eslint-disable */
const mongoose = require('mongoose');

const _schema = {
    uid: {type: mongoose.Schema.Types.ObjectId, required: true},
    gid: {type: mongoose.Schema.Types.ObjectId, required: true},
};
const schema = new mongoose.Schema(
    _schema,
    {'versionKey': false, 'usePushEach': true, 'minimize': false, 'shardKey': {'uid': 1}}
);
schema.index({'�gid': 1, 'uid': 1}, {background: false});
schema.index({'uid': 1, 'gid': 1}, {background: false});

mongoDB

test> db.test.createIndex({uid: 1, gid:1})
uid_1_gid_1
test> db.test.createIndex({uid: 1, gid:1})
uid_1_gid_1
test> db.test.getIndexes()
[
  { v: 2, key: { gid: 1, uid: 1 }, name: 'gid_1_uid_1' },
  { v: 2, key: { uid: 1, gid: 1 }, name: 'uid_1_gid_1' }
]

In my opinion, the two indexes seem to work differently in MongoDB.
why is this causing an error in mongoose? It works correctly in mongoose@8.8.x, but it behaves abnormally in mongoose@8.9.1.

[maxuai]

We had the same error for a slightly different scenario:

we had declared the unique keyword twice, once in the schema and second time in a separate index creation:

const schema = new Schema({myId: {type String, required: true, unique: true}})
schema.index({myId: 1}, {unique: true})

removing the unique from the schema declaration fixed it.

[ronenteva]

I have the same issue with this use case:

schema.index({email: 1, region: 1}, {unique: true});
schema.index({email: 1, region: 1}, {name: 'email_1_region_1-active', partialFilterExpression: {active: true}});

[ilkkaparssinen]

Error seems to be in new code for 8.9.1 (lib/schema.js):
for (const existingIndex of this.indexes()) {
if (util.isDeepStrictEqual(existingIndex[0], fields)) {
throw new MongooseError(Schema already has an index on ${JSON.stringify(fields)});
}
}

isDeepStrictEqual-function returns true for index definitionswhich have same fields in different order. Normally in javascript objects the order of properties should not matter. For mongoose/mongodb the order if items is meaningful:
(We got the error for these two indexes):
schema.index({ at: -1, url: 1 });
schema.index({ url: 1, at: -1 });

[ecofi]

When can we expect a new release with a fix? This broke all our DB connections and difficult to port back to older mongoose release. Can this have a priority?

[winterrdog]

We had the same error for a slightly different scenario:

we had declared the unique keyword twice, once in the schema and second time in a separate index creation:

const schema = new Schema({myId: {type String, required: true, unique: true}})
schema.index({myId: 1}, {unique: true})

removing the unique from the schema declaration fixed it.

ran into the same issue & this seems to have fixed it 🤝

[orgads]

I have another regression with nestjs. See nestjs/mongoose#2346.

[ecofi]

We had the same error for a slightly different scenario:
we had declared the unique keyword twice, once in the schema and second time in a separate index creation:

const schema = new Schema({myId: {type String, required: true, unique: true}})
schema.index({myId: 1}, {unique: true})

removing the unique from the schema declaration fixed it.

ran into the same issue & this seems to have fixed it 🤝

Yes, this also solved it for us. However, there is no information in the mongoose SchemaType section explaining "unique"=true creates an index too. Therefore, it would be highly appreciated to get clarification at least in the documentation, ideally with a warning info.