我如何將這個mongodb查詢轉換為C＃驅動程序？

Question

我如何為此編寫兼容的C＃代碼？

我知道我可以這樣投影：

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

但是不知道如何在查找聚合后投影姓氏以獲得作者的姓氏

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

Answer 1

試試這個

            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

假設您的作者實體嵌入在書本實體內部，那么這是一個強類型化的解決方案。 如果您的作者是被引用的實體，請告訴我，我將更新答案。

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();
        }
    }
}

上面的代碼使用我的MongoDB.Entities庫來簡化。 只需將DB.Queryable<Book>()替換為官方驅動程序的collection.AsQueryable() 。