Mongoose : find document with subdocument filter

Question

I have an User like so the id is NOT the _id)
{
  id: string;
}

Which can create files like so
{
  name: string;
  author: User;
}

I would like to get all Files where the author is a given User, but I do not know how to use the "filter" function to do that.
So currently I do
const author = await this.userModel.find({ id });
return this.filesModel.find({ author });

Is there a more efficient way to do it ?
(I use NestJS with the Mongoose integration, the syntax used is the same as the Mongoose library)
EDIT
Given the User document
{
  _id: 'OVZVIbovibiugb44'
  id: 10
}

And the Files documents
[
  { name: "1.docx", author: ObjectId('OVZVIbovibiugb44') },
  { name: "2.docx", author: ObjectId('voisbvOVISBEIVBv') },
]

I would like to use the function
findOwned(authorId = 10) {
  const author = await this.userModel.find({ id });
  return this.filesModel.find({ author });
  // But do it only with "filesModel"
}

And get, as a result,
[
  { name: '1.docx', author: 'ObjectId('OVZVIbovibiugb44') },
]

J.F. · Accepted Answer

You can use $lookup into an aggregation query to merge collections.
Also, as your id is an String and your author is an ObjectId you will need one previous stage using $toObjectId
So the query is similar to this:

$match stage (optional) to query only with documents you want. Like a filter
$project to convert id String field to ObjectId. You can user $set also.
$lookup to merge collection and the ouput is in a field called files.
$project to output only files array from the merge.

db.User.aggregate([
  { "$match": { "id": "5a934e000102030405000001" } },
  { "$project": { "id": { "$toObjectId": "$id" } } },
  { "$lookup": {
      "from": "Files",
      "localField": "id",
      "foreignField": "author",
      "as": "files" }
  },
  { "$project": { "files": 1 } }
])

Example here

Mongoose : find document with subdocument filter

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