var projection = Builders<BsonDocument>.Projection.Include("title");

db.books.aggregate(
   [
      {
         $project: {
            title: 1,
            lastName: "$author.lastName",
         }
      }
   ]
)

var project = new BsonDocument
            {
                {
                    "$project",
                    new BsonDocument
                    {
                        {"title", 1},
                        {"lastName", "$author.lastName"},
                    }
                }
            };

            var pipelineLast = new[] { project };


            var resultLast = db.books.Aggregate<BsonDocument>(pipelineLast);
            var matchingExamples = await resultLast.ToListAsync();
            foreach (var example in matchingExamples)
            {
// Display the result
            }