getPost.js

exports = function(payload, response) {
  const atlas = context.services.get('mongodb-atlas');
  return atlas
    .db('apitutorial')
    .collection('posts')
    .aggregate([
      {
        $match: {
          _id: BSON.ObjectId(payload.query.id),
        },
      },
      {
        $project: {
          title: 1,
          desc: '$description',
          userId: 1,
        },
      },
      {
        $lookup: {
          from: 'users',
          let: { userId: '$userId' },
          pipeline: [
            {
              $project: { name: 1, age: 1 },
            },
          ],
          as: 'userId',
        },
      },
    ])
    .toArray();
};