How to insert an array as a new document after the .aggregation in mongodb?

Question

I wanner insert a var object as a new document after aggregation operation in MongoDB. I try this method:

    var postUpvote = db.collection.aggregation([
    {$limit:1}
    ]).toArray()

    db.collection.aggregation([
    ...
    ...
    ]).insert(postUpvote[0])

But db.collection.aggregation(...).instert() is not a function. The second aggregation result is just one document. And I also try to use $addFields to insert the postUpvote to the document, but I can't take this inserted file out of the document into a new document.

{
    "_id" : 1671,
    "CommentUserId" : 1671,
    "CommentUserUpVote" : 2644,
    "PostUserId" : 1671,
    "PostUserVote" : 2644
}

The PostUserId and PostUserVote is the var postUpvote. The result I want is

{
    "_id" : 1671,
    "CommentUserId" : 1671,
    "CommentUserUpVote" : 2644,
}
{
    "_id": //Whatever, I don't care
    "PostUserId" : 1671,
    "PostUserVote" : 2644
}

Answer 1

You can use $out aggregation as a last step in your pipeline. But it replaces the output collection completely if it already exists. If you want to avoid it try $merge aggregation. It can incorporate results (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) into an existing collection.