How to clone a document instance

[saveman71]

Let's say I have a function that takes a Document instance in parameter that may or may not have some populated paths.

I'd like this function to always work a depopulated version of the document, so I could call .depopulate() on the document. But this would have the side effect of depopulating the paths for the caller context as well.

How do I clone / deep copy a Document instance to do some work on it without side effects?

[vkarpov15]

There are a couple of possibilities that may work depending on your use case.

Easiest way would be to just do const clonedDoc = new MyModel(...doc.toObject()) or const clonedDoc = MyModel.hydrate(doc.toObject()), which gets you a new document with all the same properties as doc. You can then depopulate and do whatever you need to.

The downside is that you lose change tracking, so clonedDoc wouldn't be aware of what paths in doc are modified. But if you don't intend to do clonedDoc.save(), this could work.

[saveman71]

The use case is to do changelog style tracking, so I'm comparing objects before and after modification, and I don't save them, but also I don't want an "in-between" population generating changes.

So I do a toObject() on the depopulated object, which would look like:

track(doc) {
  const cloned = new this._model({...doc.toObject()});
  cloned.depopulate();
  this._original = cloned.toObject({ getters: false, virtuals: false });
}

It's a bit weird to call toObject twice (just to depopulate), but I get why it's needed.

An additional option in the toObject() to not include populated paths would fit my use case as well. Is this something that exists / would be welcomed as a feature?

A "clone()" method would make that cleaner but I assume it would essentially be the same logic behind the scenes? (also quite hard to add a clone() method without breaking changes)

[vkarpov15]

Re: option to not include populated paths in toObject(), we already have that. const cloned = new this._model(doc.toObject({ depopulate: true })). No need to use spread operator, toObject() clones doc's data.

It would be similar logic behind the scenes, the difference is that we'd also copy over change tracking info. But if you don't need a fully hydrated copy, you just need a POJO for diffing, you don't need a clone() method. toObject({ depopulate: true }) should be what you need.

[saveman71]

Okay, I used the spread operator because you included it in your first answer, so maybe it's best edit it then?

But yes, thanks! I wasn't aware of the depopulate option, it's exactly what I need.

track(doc) {
  this._original = cloned.toObject({ getters: false, virtuals: false, depopulate: true });
}