mongodb reverse populate

/**
 * With `authors` schema: { _id, name } and `books` schema: { _id, name, authorId },
 * select all authors with all their books.
 */

// Simple way
db.authors.aggregate([
  {
    $lookup: {
      from: "books",
      localField: "_id",
      foreignField: "authorId",
      as: "books",
    },
  },
]);

// Equivalent, but more verbose; supports more complex cases
db.authors.aggregate([
  {
    $lookup: {
      from: "books",
      let: { id: "$_id" }, // $_id here is author._id, used in the pipeline below as $$id
      pipeline: [
        {
          $match: {
            $expr: {
              $and: [
                { $eq: ["$authorId", "$$id"] }, // $authorId here is permision.authorId
                // possible other conditions here
              ],
            },
          },
        },
      ],
      as: "books",
    },
  },
]);