fix(populate): merge instead of overwrite when match is on _id

[vkarpov15]

Fix #12834

Summary

#12834 points out that match on _id overwrites rather than merges. Which is not a very good API design, because any custom match that specifies _id will overwrite the _id filter specified by populate(). This PR fixes that.

Examples

[AbdelrahmanHafez]

LGTM, thanks! 👍
I have no idea how I survived not being bitten by this bug all those years. 😄

[vkarpov15]

@AbdelrahmanHafez do you find yourself using match with _id? I've never done that before, although I can see why it would be tempting.

[AbdelrahmanHafez]

Now that I think about it, I don't think I ever did use match with _id, but I can see a scenario where I would want to find an intersection/subset between documents coming from one place and matching them in a populate.

[yassine-cc]

Hey @vkarpov15 , i'm the one who created the issue related to this, thank you for fixing it.

I just want to point out that the test case in the PR wouldn't catch this bug as only the ids that are present get populated
and so the issue wouldn't affect the length of the populated array inside the document.

There is a difference between the query results returned by the db and the result that mongoose produces.

The bug is more about the performance impact of getting a huge query result size back from the db (huge memory usage), instanciating every item in the array, and then iterating over them to match them with the ids inside the array to populate

[vkarpov15]

@yassine-cc can you please clarify the issue you're seeing? I unfortunately don't understand what you mean by "only the ids that are present get populated and so the issue wouldn't affect the length of the populated array inside the document"