mongodb apply sort to lookup results

Question

If I have a user and post collection

{\"_id\": 1, \"name\": \"User 1\"}
{\"_id\": 2, \"name\": \"User 2\"}

{\"_id\": 1, \"title\":         


        

Answer 1

I solved the duplicates using $group and $first

db.getCollection('user').aggregate([
    {$lookup: {from: "post", localField: "_id", foreignField: "userId", as: "post"}},
    {$unwind: { path: "$post", preserveNullAndEmptyArrays: true }},
    {$sort: {"post.createdAt": -1}},
    {$group: {"_id": "$_id", "name": {$first: "$name"}, "post": {$first: "$post"}},
    {$project: {"_id": 1, "name": 1, post": 1}}
])

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