nested $lookup returns array instead of object

MongoDB Atlas
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1

here are my Schemas :

const property = mongoose.Schema({
  _id: mongoose.Schema.Types.ObjectId,
  reviews: [{ type: mongoose.Schema.Types.ObjectId, ref: "Review" }],
});

const review = mongoose.Schema({
  _id: mongoose.Schema.Types.ObjectId,
  text: { type: String, default: null },
  author: { type: mongoose.Schema.Types.ObjectId, ref: "Author" },
});

const author = mongoose.Schema({
  _id: {type: mongoose.Schema.Types.ObjectId, default: new mongoose.Types.ObjectId()},
  name: { type: String, default: "" },
});

Each Property has a review and each Review has an Author,

Here my code its working I have a nested $lookup for reviews and author but the Author is return as Array I want it as object :

Property.aggregate([
        {
          $match: {
            _id: new mongoose.Types.ObjectId(propertyId),
          },
        },
        {
          $lookup: {
            from: "reviews",
            let: {
              reviews: "$reviews",
            },
            pipeline: [
              {
                $match: {
                  $expr: {
                    $and: [{ $in: ["$_id", "$$reviews"] }],
                  },
                },
              },
              {
                $lookup: {
                  from: "authors",
                  let: {
                    author: "$author",
                  },
                  pipeline: [
                    {
                      $match: {
                        $expr: {
                          $and: [{ $eq: ["$_id", "$$author"] }],
                        },
                      },
                    },
                    { $unwind: "$author" },
                  ],
                  as: "author",
                },
              },
            ],
            as: "reviews",
          },
        },
      ]).exec();

Here is what I got as result :

{
  _id : 1,
  reviews: [{
    _id: 1,
    text: 'nice view',
    author: [{
      id: 1,
      name: 'Victor'
    }]
  }]
}

you can see that the Author is return as Array I want it as object :

2

A $lookup always returns an array. If you know for there is only 1 element you may $set or $addField to replace the array with its first element with

https://www.mongodb.com/docs/v5.0/reference/operator/aggregation/first-array-element/.