MongoDB - findOne with nested subdocuments and projection - node.js

Question

I am currently using the code below in node.js to find and return data on various nesting levels from a mongo database. I'd like to add another layer of nesting (as mentioned in #3).
Collection:
[
{
"title": "Category A",
"link": "a",
"items": [
{
"title": "Item C",
"link": "a-c",
"series": [
{
"title": "Item C X",
"link": "a-c-x"
},
{
"title": "Item C Y",
"link": "a-c-y"
},
]
},
{
"title": "Item D",
"link": "a-d"
}
]
},
{
"title": "Category B",
"link": "b"
}
]
The query:
const doc = await ... .findOne(
{
$or: [
{ link: id },
{ "items.link": id },
{ "items.series.link": id }
],
},
{
projection: {
_id: 0,
title: 1,
link: 1,
items: { $elemMatch: { link: id } },
},
}
);
Intended results:
(works) if link of the document is matched,
(works) there should only be an object with the title and link returned
e.g.
value of id variable: "a"
expected query result: { title: "Category A", link: "a"}
(works) if items.link of subdocument is matched,
(works) it should be the same as above + an additional element in the items array returned.
e.g.
value of id variable: "a-c"
expected query result: { title: "Category A", link: "a", items: [{ title: "Item C", link: "a-c" }]}
(works) if items.series.link of sub-subdocument is matched
(struggling with this) it should return the same as in 2. + an additional element inside the matched items.series
e.g.
value of id variable: "a-c-y"
expected query result: { title: "Category A", link: "a", items: [{ title: "Item C", link: "a-c", series: [{ title: "Item C Y", link: "a-c-y" }]}]}
current query result: The whole Category A document with all sub-documents
Questions:
a.) How do I modify the projection to return the expected output in #3 as well?
b.) Is the approach above sound in terms of reading speed from a denormalized structure? I figured there'd probably need to be indexes on link, items.link and items.series.link as they are all completely unique in the document, but maybe there is a way to achieve the above goal with a completely different approach?

Answer 1

Ended up with going half-way via mongodb and get the full item for both - when the item link is matched and the series link is matched:
projection: {
_id: 0,
title: 1,
link: 1,
items: { $elemMatch: { $or: [
{ link: id },
{"series.link": id }
]}},
}
After that javascript filters the series array to see if the series is matched:
doc?.items?.[0]?.series?.find(item => item.link === id)
if the js is truthy (returns an object) we matched a series, if there is a doc, but the js is falsy we matched an item result.
Although not a full mongodb solution and there is definitely room for improvement the above seems to achieve the end goal to be able to distinguish between category, item and series results.