How to add more fields in mongoDB database schema based on other field

const mongoose = require("mongoose");

const userSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true,
  },
  email: {
    type: String,
    required: true,
  },
  password: {
    type: String,
    required: true,
  },
  role: {
    type: String,
    default: "user",
  },

});

module.exports = mongoose.model("USER", userSchema);

For example this is user schema. Now if while registering a user, then if the user.role == "doctor" then the schema should be

const mongoose = require("mongoose");

const userSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true,
  },
  email: {
    type: String,
    required: true,
  },
  password: {
    type: String,
    required: true,
  },
  role: {
    type: String,
    default: "user",
  },
  fees: {
    type: Number,
    default: 1000,
  },
  specialization: {
    type: String,
    default: "Physicist",
  },
  location: {
    type: String,
    default: "India",
  },
});

module.exports = mongoose.model("USER", userSchema);

or if the user.role == "user" then the schema should be normal original one without the fields such as fees, specializatoin, location

I tried using mongoose.pre function but it didnt work. This is example

userSchema.pre("save", function (next) {
  if (this.role === "doctor") {
    this.fees = 1000; 
    this.specialization = "Physicist"; 
this.location= "India"; 
  }
  next();
});

To achieve conditional schema fields based on the role property in the user schema, you can make use of Mongoose's discriminators.
Discriminators allow you to define different schemas based on a common parent schema.

Here's how you can modify your code to implement discriminators:

const mongoose = require("mongoose");

const userSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true,
  },
  email: {
    type: String,
    required: true,
  },
  password: {
    type: String,
    required: true,
  },
  role: {
    type: String,
    default: "user",
  },
});

const User = mongoose.model("User", userSchema);

const doctorSchema = new mongoose.Schema({
  fees: {
    type: Number,
    default: 1000,
  },
  specialization: {
    type: String,
    default: "Physicist",
  },
  location: {
    type: String,
    default: "India",
  },
});

const Doctor = User.discriminator("Doctor", doctorSchema);

module.exports = { User, Doctor };

In the above code, we define the base User schema with common fields like name, email, password, and role. Then, we create a new schema called doctorSchema that includes the additional fields specific to the doctor role.

Using User.discriminator("Doctor", doctorSchema), we create a discriminator model called Doctor based on the User model. This will allow us to create instances of the Doctor schema when the role property is set to "doctor".

Now, when registering a user, you can check the role value and create a user accordingly:

const { User, Doctor } = require("./userSchema");

// Example usage when registering a user
const registerUser = async (userData) => {
  let newUser;
  
  if (userData.role === "doctor") {
    newUser = new Doctor(userData);
  } else {
    newUser = new User(userData);
  }
// don't forget to add error handling for this.
await newUser.save()
}