For my project, I am trying to allow users to use multiple emails or phone numbers.

My schema look like this:

const username = {
    type: String,
    trim: true,
    sparse: true,
    unique: true,
    lowercase: true
}

var userSchema = Schema({
  name : { type : String },
  emails: [username],
  phones: [username],
})

I used sparse because I want users to use email/phone when signing up.

Am getting the following error when a second user tries to signup without providing a phone number.

ERR MongoServerError: E11000 duplicate key error collection: app.users index: phones_1 dup key: { phones: undefined }

{

    "_id" : ObjectId("6215c3d9ec72c9f46b23f6d3"),
    "name" : "Test User",
    "emails" : [
            "test@gmail.com"
    ],
    "phones" : [ ],

}

How can I setup unique sparse index in nested array?