Duplicate key error while using _id from another collection

Question

I have 2 collections so far.
The first is a 'Users' collection (That works well), and the other is a 'Rooms' collection (both created for a chat application).

I want every room to have a "users" array that will contain the user._id of every user that is in that room,
meaning I should be able to put the same user._id (from the user collection) in every one of the rooms right?

After creating a room successfully with 2 user._ids in the "users" array,
I tried making another one using one of the user._ids I used in the first room.
Then I got this error:

MongoError: E11000 duplicate key error collection: ratchat.rooms index: users_1 dup key:
{ users: "5fe08d452f34530e641d8f8c" }

After checking with a debugger I've found that the error occurs only when I use a user._id that is already used in another room's "users" array.
The only thing I could think of that could cause this problem is the Room schema,
maybe there's something I missed while reading the docs...

At first my Room schema looked like this:

  const roomSchema = new mongoose.Schema({
  users: [String],
  hasLeft: [String],
  isGroup: { type: Boolean, default: false },
  },
});
const Room = mongoose.model("Room", roomSchema);

Then I thought maybe mongoDB needs to know that the ObjectIds that are in the users array are just references to another collection:

  const roomSchema = new mongoose.Schema({
  users: [{ type: mongoose.Schema.Types.ObjectId, ref: "User" }],
  hasLeft: [String],
  isGroup: { type: Boolean, default: false },
  },
});
const Room = mongoose.model("Room", roomSchema);

No luck so far...

Answer 1

{autoIndex: false}

After research, I have found the reason for this error:
mongoose automatically creates indexes,
this is not only a duplicate error issue, but can cause a significant performance impact later in production.

According to mongoose docs, you can easily disable this behavior by setting the autoIndex option of your schema to false, or globally on the connection by setting the option autoIndex to false.

  mongoose.connect('mongodb://user:pass@localhost:port/database', { autoIndex: false });
  // or
  mongoose.createConnection('mongodb://user:pass@localhost:port/database', { autoIndex: false });
  // or
  animalSchema.set('autoIndex', false);
  // or
  new Schema({..}, { autoIndex: false });

Don't forget to drop the entire collection before trying again

Because the collection is already indexed, emptying it completely won't work.
You have to drop the entire collection.