How to do two-level aggregation in a query?

Question

MongoDB

Good afternoon!
There are currently 3 collections, with this format:

let item = {
    _id: "...",
}

let itemGroup = {
    _id: "..."
}

let relation = {
    item_id: "...",
    group_id: "..."
}

Previously, this relationship was one-to-one and the item had a group_id, through which we got all the fields using an aggregation of this type:

group.aggregate([
        {
          $project: {
            _id: {
              $toString: '$_id',
            },
            title: 1,
            order: 1,
          },
        },
        {
          $lookup: {
            from: 'fields',
            localField: '_id',
            foreignField: 'group_id',
            as: 'fields',
          },
        },
      ]);

I can't figure out how to make a two-level aggregation with one request - first get all the links, and through it all the elements. Now I divided it into two requests, but the understanding that this can be done by one directly does not let me sleep)

Answer 1

I'm not completely into everything, but it seems to me that you need this operator https://docs.mongodb.com/manual/reference/operator...
I.e. you will have the following pipeline: $project, $lookup, $unwind, $lookup.