MongoDB 聚合 $lookup $match

Question

my Connection collection have ObjectId Array field named authors. I want to concat all first names and last names from author arrays.Right now I get name from all users in collection. Can't get $match aggregation to work. Thank you!

Connections Schema

const connectionSchema = new mongoose.Schema({
  authors: [{ type: mongoose.Schema.Types.ObjectId, required: true }],
  messages: [messageSchema]
});

Problem in code

   const connections = await Connection.aggregate([

    {
      $lookup: {
        from: "users",
        let: { users: "users" },
        pipeline: [
          { $match: { _id: { $in: ["_id", "$authors"] } } },
          {
            $project: {
              name: {
                $concat: ["$firstname", " ", "$lastname"]
              }
            }
          }
        ],
        as: "userName"
      }
    },

Answer 1

You have authors field in connection schema. So instead use authors in let variable to get use in users pipeline { "$in": ["$_id", "$$authors"] } .

const connections = await Connection.aggregate([
  {
    "$lookup": {
      "from": "users",
      "let": { "authors": "$authors" },
      "pipeline": [
        { "$match": { "$expr": { "$in": ["$_id", "$$authors"] } } },
        {
          "$project": {
            "name": {
              "$concat": ["$firstname", " ", "$lastname"]
            }
          }
        }
      ],
      "as": "userName"
    }
  }
])