Persisting non schema values between hooks

[bshamblen]

Previously (similar to #2363) I was storing a copy of the existing doc in the post init hook, then in the pre save hook I compared the original doc with the updated doc (if not new), then in the post save hook I had logic to send a notification of the changes.

Before I upgraded to 4.0, the code below used to work (I don't recall which version of 3.8 I was using), but now I receive the error message Object #<EventEmitter> has no method 'toObject' when trying to copy the current object. I've been trying to come up with a workable solution, but I can't see a way to persist the original document through to the post save event.

My original code (3.8.x)

schema.post('init', function() {
    this._original = this.toObject();
    this._changes = []; //array of objects with the path, new and old values. Populated by `observe` function in pre `save` hook
});

schema.pre('save', function(next) {
    //other logic here. Cut out for this example..

    this.observe(); //compares the current doc with the original and stores the new and old values in `this._changes` array.
    next();
});

schema.post('save', function(doc) {
    if (this._original && this._changes.length > 0) {
        //send web hook notification, with changes
    } else {
        //send web hook notification, with new document
    }

    //reset the values, in case the document is saved again.
    this._original = null;
    this._original = this;
    this._changes = [];
});

The problem with the new post init architecture is that when I add the doc parameter to the hook function I can get the doc and add a copy to itself, but that copy will no longer be available in the pre or post save hooks.

Changed to (4.0.x)

schema.post('init', function(doc) {
    doc._original = doc.toObject();
    doc._changes = [];
});

Later, in the pre save hook, this._original is undefined, even for documents that we returned from the database. Any suggestions on how to reliably save a copy of the original document and have it survive the document lifecycle.

[vkarpov15]

This issue is another manifestation of #2949 and #2925, accidental regression in 4.0.2 that occurred when we theoretically brought back the ability to do:

schema.post('init', function() {
    this._original = this.toObject();
    this._changes = []; //array of objects with the path, new and old values. Populated by `observe` function in pre `save` hook
});

You should be able to do that again in 4.0.3, the below script works for me on 4.0.3:

var mongoose = require('mongoose');
mongoose.set('debug', true);
var util = require('util');
var assert = require('assert');

mongoose.connect('mongodb://localhost:27017/gh2952');

var UserSchema = new mongoose.Schema({
  name: { type: String },
});

UserSchema.post('init', function(doc) {
console.log('post init');
  doc._original = doc.toObject();
});

var User = mongoose.model('user', UserSchema);

User.create({ name: 'val' }, function(error) {
  assert.ifError(error);
  User.findOne({}, function(error, u) {
    console.log(require('util').inspect(u._original));
    process.exit(0);
  });
});