我如何将这个mongodb查询转换为C＃驱动程序？

Question

How can i write a compatible c# code for this?

I know i can do projection like this:

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

But no idea how to project the last name to get the author's last name after a lookup aggregation

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

Answer 1

Try this one

            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 
            }

Answer 2

assuming your author entity is embedded inside the book entity, here's a strongly typed solution. if your author is a referenced entity, let me know and i'll update my answer.

using MongoDB.Entities;
using System;
using System.Linq;

namespace StackOverflow
{
    public class Program
    {
        public class Book : Entity
        {
            public string Title { get; set; }
            public Author Author { get; set; }
        }

        public class Author
        {
            public string FirstName { get; set; }
            public string LastName { get; set; }
        }

        private static void Main(string[] args)
        {
            new DB("test");

            (new Book
            {
                Title = "a book title goes here",
                Author = new Author
                {
                    FirstName = "First Name",
                    LastName = "Last Name"
                }
            }).Save();

            var res = DB.Queryable<Book>()
                        .Select(b => new
                        {
                            Title = b.Title,
                            LastName = b.Author.LastName
                        }).ToArray();

            foreach (var b in res)
            {
                Console.WriteLine($"title: {b.Title} / lastname: {b.LastName}");
            }

            Console.Read();
        }
    }
}

the above code is using my library MongoDB.Entities for brevity. simply replace DB.Queryable<Book>() with collection.AsQueryable() for the official driver.