Playlists Aggregation

playlists.js

function getPlaylist(userId, playlistId) {
  return database.playlists.aggregate([{
    $match: {
      _id: ObjectId(playlistId),
      user_id: userId
    }
  }, {
    $unwind: {
      path: "$audios",
      preserveNullAndEmptyArrays: true
    }
  }, {
    $lookup: {
      from: "audios",
      localField: "audios",
      foreignField: "_id",
      as: "audios"
    }
  }, {
    $unwind: {
      path: "$audios",
      preserveNullAndEmptyArrays: true
    }
  }, {
    $sort: {
      "audios.created_at": -1
    }
  }, {
    $group: {
      _id: "$_id",
      title: {
        $first: '$title'
      },
      description: {
        $first: '$description'
      },
      isPrivate: {
        $first: '$isPrivate'
      },
      updated_at: {
        $first: '$updated_at'
      },
      audios: {
        $push: "$audios"
      },
      type: {
        $first: '$type'
      }
    }
  }])
}