How to perform the pre save hook functionality when updating

Question

In my document I have some properties called height, weight, BMI. I used the pre save hook to calculate and store the BMI on document create or save. But I want to Update the BMI whenever the height or weight is updated.

//this is the pre save hook that I used

studentSchema.pre('save', function (next) {
  this.bmi = calcBMI(this.height, this.weight);
  next();
});

//BMI Calculating Function

exports.calcBMI = (height, weight) => {
  const heightInMeters = height / 100;
  const bmi = (weight / heightInMeters ** 2).toFixed(2);
  return bmi;
};

This function works well when I create a new student.

Answer 1

you have to use mongoose setters in schema like:

const studentSchema = new Schema({
  weight: {
    type: Number,
    set: () => this.bmi = calcBMI(this.height, this.weight);
  },
...same with height
});

official mongoose page: mongoose setters page

Answer 2

You can use isModified() helper function on a document property and check if it's change or not

studentSchema.pre('save', function (next) {
  const std = this;
  if(std.isModified("height") || std.isModified("weight")){
   this.bmi = calcBMI(this.height, this.weight)
}
  next();
});