mongoose-unique-validator detects all inputs as already existed - node.js

Question

I am trying to create a unique email value in MongoDB. I used mongoose-unique-validator to implement that but resulted in error claiming that my unique emails that I just inputted are already existed.
This is the error message I received from trying to input unique email.
"message": "User validation failed: email: Error, expected email to be
unique., _id: Error, expected _id to be unique."
They said the Email and _id are not unique. But.., _id is an auto-generated value to be unique in MongoDB meaning that it should not be detected as a duplicated value.
I am not sure if that was caused by my own implementation so I would look forward to see any assumption or ways to debug to the root cause of this too. I tried restarting from fresh Mongo DB and manually inputting uniquely via Postman but still no hope.
These are a part of the codes that might be related to the data creation on MongoDB
UserModel.js
var uniqueValidator = require('mongoose-unique-validator');
const { Schema } = mongoose;
const UsersSchema = new Schema({
email: { type: String, unique: true, required: true},
hashedPassword: String,
salt: String,
});
UsersSchema.plugin(uniqueValidator, { message: 'Error, expected {PATH} to be unique.' });
UsersSchema.set('autoIndex', false);
Server.js ---- /api/users/register
const finalUser = new Users(user);
finalUser.setPassword(user.password);
finalUser.save((err, data) => {
if (err) {
return res.status(400).json(err)
}
return res.json({ user: finalUser.toAuthJSON() })
})
Additional Information
I tried this solution from Sritam to detect another email with the same value but it still claims that the inputted email is already existed.
UsersSchema.path('email').validate(async (value) => {
const emailCount = await mongoose.models.User.countDocuments({email: value });
return !emailCount;
}, 'Email already exists');
"message": "User validation failed: email: Email already exists"

Answer 1

You can use validateModifiedOnly option in Document.save(). _id and email fields will never be validated unless they are modified. The code should look like this:
finalUser.save({ validateModifiedOnly: true }, (err, data) => {
if (err) {
return res.status(400).json(err)
}
return res.json({ user: finalUser.toAuthJSON() })
})

Answer 2

Found the issue and solution!
They are acting weird like in the question because the model was not initialized.
I must perform Schema.init() before performing any model validation.
The solution is to add UsersSchema.init().then(() => {...your operation})
Now Server.js ---- /api/users/register should look like this.
Users.init().then(() => { // where Users is my UsersModel
finalUser.save((err, data) => {
if (err) {
return res.status(400).json(err)
}
return res.json({ user: finalUser.toAuthJSON() })
})
})
Hope this helps other developers who experience similarly odd error!