How can we add the one collection id to another collection in mongoose?

Here I want to show the todos created by the the user which is in params and also want to show the id of user in each todos array object.How Can I achieve this

Here is my userSchema

const mongoose = require('mongoose')
const userSchema = new mongoose.Schema({


 name :{
        type: String,
        required: true

    },
    email:{
   type: String,
   required: true,
unique: true
    },


phone:{
    type: String,
    required: true,
},
todos: [{
    type: mongoose.Schema.ObjectId,
    ref: 'Todo',

},
]})

userSchema.pre(/^find/ , function(next)     
this.populate('todos')
next()
 })

const User = mongoose.model('User' , userSchema)

module.exports =User

This is the getUserById code

 
    const user= await User.findById(req.params.id).populate('todos')                  
    if(!user){
        return next(new AppError(`Cant find this user from that ID ${req.originalUrl} :(` ,404))
    }
    res.status(200).json({
        status: 'sucess',
        data:{
      user
        }
    })
})

Thanks in advance :)