strip option to Model.hydrate(...)

[AbdelrahmanHafez]

In #15638 we created a virtuals: boolean option to Model.hydrate(...) that allows people to hydrate virtual paths to aggregated results, the virtual path mentioned in #15627 doesn't have a particular type, it's the equivalent of "Mixed" which I assume is just added for this purpose. IMO the "virtual" path here is not special at all, it's just added to enable adding an arbitrary field to be hydrated.

I wonder if we should have a strip option that defaults to true, this option would dictate how Mongoose should handle fields that are not explicitly defined in the schema. I can now think of many cases where I would've liked to cast aggregation results without stripping the additional fields by using Model.hydrate(rawResult, null, { strip: false });.

Thinking out loud here: If we agree to implement that, we can even take it further and make Model.aggregate(...) accept a cast: boolean, strip: boolean, that would enable people to do cast: true, strip: false, which would be very useful for casting aggregation pipelines that mostly add/remove fields without fundamentally changing the shape of the document, while keeping the non-defined fields as is. I'd even think it's worth discussing making this the default behavior of aggregation pipelines, I can see a couple of arguments for both with and against this.

I'd be happy to implement these features if we agree to add them.

What do you think? @vkarpov15 @hasezoey

[vkarpov15]

Check out PR #15944