fix populate called with lean (gh-14794)

[MohOraby]

Summary

in #14794 due to being called with lean() there's no _doc object so that throws an error

another issue here is documents with duplicate foreignField so the rawDocs[key] already exists and its value is being overwritten.

this change shouldn't affect (gh-14759) and its test case still runs successfully.

another solution would be to check for rawDocs[key]._id and val._doc first using the following statement instead

(rawDocs[key]._doc && String(rawDocs[key]._doc._id)) !== (val._doc && String(val._doc._id))) 

[vkarpov15]

A couple of quick changes, #14759 made us very wary of cases where we may accidentally be checking for quality on _id after getters applied, so we want to avoid undoing the val._doc._id changes in cases where we know val instanceof Document.

Thanks for finding the issue

[vkarpov15]

This change isn't necessary. L4462 will never run if lean.

[vkarpov15]

Can we expand this change to check if rawDocs[key] and val are instanceof Document, and, if so, use val._doc._id, otherwise val._id?

[MohOraby]

Ok, for this would you prefer it be a ternary operator inside the else if check like this

(rawDocs[key] instanceof Document ? String(rawDocs[key]._doc._id) : String(rawDocs[key]._id)) !== (val instanceof Document ? String(val._doc._id) : String(val._id))

which looks kind of long and hard to read or move them to constant above the if check like this

const rawDocId = rawDocs[key] instanceof Document ? rawDocs[key]._doc._id : rawDocs[key]._id;
const valDocId = val instanceof Document ? val._doc._id : val._id;
// rest of the code
String(rawDocId) !== String(valDocId)

both are fine and would do the same thing, I just don't wanna keep pushing commits since the CI would rerun

[MohOraby]

I went for the the first approach currently, just to move things forward, if it needs refactoring please let me know

[vkarpov15]

See previous comments, I accidentally submitted review with "Comment" selected rather than "Request changes"

[vkarpov15]

Thanks 👍