How to check modified fields in pre/post update hook in Mongoose.js

If you want to know what fields you are modifying, then you need to issue an update command by calling save:

Tank.findById(id, function (err, tank) {  if (err) return handleError(err);  tank.set({ size: 'large' });  tank.save(function (err, updatedTank) {    if (err) return handleError(err);    res.send(updatedTank);  });});

This way pre-save hook will be invoked and you will be having access to:

Document.prototype.modifiedPaths()

because this in pre-save hook refers to the document:

TankSchema.pre('save', function (next) {  // ...  this.modifiedPaths();  // ...});

On the other hand, you will not be able to achieve the same result when issuing an update command by calling update:

Tank.update({ _id: id }, { $set: { size: 'large' }}, callback);

because when calling update, document hooks (e.g. pre-save, post-save) are not being executed at all. Instead, query hooks are being executed (e.g. pre-update, post-update) in that case. And the issue with query hooks is that this inside of them does not reference the document, so this.modifiedPaths === undefined

schema.post('update', function (doc) {  // `this` refers to model.Query  // `doc` refers to CommandResult });

try it:

 schema.post('update', function () {     const modifiedFields = this.getUpdate().$set;     // ... });

There is no direct way of doing it. There is a variable called isNew which is being tracked by mongoose to check if a new document is created.

see Document#isNew for information on Document#isNew

However, you can created your own tracker to check in post save hook to identify wether document is updated or not.

schema.pre('save', function (doc) {    this.isDocUpdated = false;    if (this.isModified()) {        this.isDocUpdated = true;    }});schema.post('save', function (doc) {    if (this.isDocUpdated) {        console.log('post save hook called');    }});