Querying a parent child referencing in mongoose

Question

I have the comment model as:

const commentSchema = new Schema(
  {
    content: String,
    score: Number,
    replyingTo: { type: mongoose.Types.ObjectId, ref: "Comment" },
  }
);
const Comment = mongoose.model("Comment", commentSchema);

I want to return a query like below. how can I do that?

{
content:"s sample content for comment",
score:12,
replies:["644914e42dcb3e56cf9e8f4b" , "675fd4914e42dcb3edfr8f9e8f4b" ,"234fd4914e42dcb3edfr8dfr8f4b"]
}

I guess with virtual properties or aggregation framework can we do that

Answer 1

you can get the array of replies by doing a self $lookup. After that it is just a $project. Since your output has stringified object ids you might have to $map through the array and convert each $toString. If this is not needed you can just project replies: "$replies._id"

db.comments.aggregate([
  {
    $lookup: {
      from: "comments",
      localField: "_id",
      foreignField: "replyingTo",
      as: "replies"
    }
  },
  {
    $project: {
      _id: 0,
      content: 1,
      score: 1,
      replies: {
        $map: {
          input: "$replies",
          as: "reply",
          in: {
            $toString: "$$reply._id"
          }
        }
      }
    }
  }
])

playground