Mongoose: How to set the value of a field based on another field?

I am trying to have a field in the schema based on another.

I found this solution:

const PostSchema = new Schema({
    title: { type: String, required: true },
    titleUpperCase: { type: String }
    content: { type: String, required: true },
    creation: { type: Date, default: Date.now() }
})
PostSchema.pre('save', (next) => {
    this.titleUpperCase = this.title.toUpperCase()
    next()
})

In the above code, I want the value of the titleUpperCase field to be based on the title field
but the problem is i give an error says: TypeError: Cannot read properties of undefined (reading 'title')

>Solution :

Because you are using an arrow function, the "this" keyword is not what you want it to be. Basically, whenever you are using mongoose hooks like a pre save hook in this case, you should always use a normal function for a callback.

 PostSchema.pre('save', function(next) {
    this.titleUpperCase = this.title.toUpperCase()
    next()
})

I believe this should work.