Fix dynamic virtual populate on sub-documents

[Hybrid-Force]

Fix #12363

Summary
Apply dynamic ref getter of virtual population on sub-documents instead of the parent document.

Mongoose supports dynamic virtual populate via ref and refPath options. However, the ref function is called on the parent document, and refPath is relative to the parent document. It does not make much sense for dynamic virtual populates defined on nested schemas, because the child schema has no knowledge of the parent schema.

After this patch, ref is called on sub-document, and refPath is relative to sub-document instead of the parent document.

[vkarpov15]

I took a closer look and the distinction that refPath is relative to the top-level document rather than the subdocument is intentional. Paraphrasing from this comment: if refPath is relative to the subdocument, it is impossible to make refPath reference a field outside of the subdocument (like a top-level field) without introducing new syntax. Whereas with ref(), you can always call this.ownerDocument() to get the top-level document.

I do like the fix that ref() on virtuals is now called on the subdocument rather than the top-level document. But refPath should still be relative to top-level document. Is that possible?

Also, it looks like these changes only affect virtual populate. So it looks like refPath is still relative to top-level document for conventional populate.

[Hybrid-Force]

I do like the fix that ref() on virtuals is now called on the subdocument rather than the top-level document. But refPath should still be relative to top-level document. Is that possible?

Sure, I will update the patch.

[hasezoey]

seeing as this PR has been open for quite some time with no activity and changes were requested, should this PR be closed?

(from what it looks like this PR was targeting ~6.7, and now mongoose is at 8)