MongoDB 中的关系（使用 .Net）

Question

在 mongoDB 中是否有使用 .Net 创建某种等同于“SQL-Join”的东西？ 我已经阅读了有关关系的 MongoDB 文档 (https://docs.mongodb.com/manual/tutorial/model-referenced-one-to-many-relationships-between-documents/)。据我了解，您只需通过引用它们的 ID 来添加关系。然而..这是否也意味着对于每一个关系你还需要做一个额外的查询？..

Answer 1

考虑如下最简单的关系：

db.publishers.save({
    _id: "oreilly",
    name: "O'Reilly Media",
    founded: 1980,
    location: "CA"
})

db.books.save({
    _id: 123456789,
    title: "MongoDB: The Definitive Guide",
    author: [ "Kristina Chodorow", "Mike Dirolf" ],
    published_date: ISODate("2010-09-24"),
    pages: 216,
    language: "English",
    publisher_id: "oreilly"
})

在 MongoDB 中，您可以使用 $lookup 运算符在一个查询中从两个集合中获取数据：

db.books.aggregate([
    {
        $lookup: {
            from: "publishers",
            localField: "publisher_id",
            foreignField: "_id",
            as: "publisher"
        }
    }
])

返回：

{ 
    "_id" : 123456789, 
    "title" : "MongoDB: The Definitive Guide", 
    "author" : [ "Kristina Chodorow", "Mike Dirolf" ], 
    "published_date" : ISODate("2010-09-24T00:00:00Z"), 
    "pages" : 216, 
    "language" : "English", 
    "publisher_id" : "oreilly", 
    "publisher" : [ { "_id" : "oreilly", "name" : "O'Reilly Media", "founded" : 1980, "location" : "CA" } ] 
}

使用 MongoDB .NET 驱动程序，您可以使用 LINQ 语法和 join 运算符，这些运算符将被转换为 $lookup：

var books = db.GetCollection<Book>("books");
var publishers = db.GetCollection<Publisher>("publishers");

var q = from book in books.AsQueryable()
        join publisher in publishers.AsQueryable() on
            book.publisher_id equals publisher._id
        select new
        {
            book,
            publisher = publisher
        };

var result = q.ToList();

用$unwind 翻译成$lookup，这样你就得到一个publisher 对象而不是数组

Answer 2

如果您不介意使用库，MongoDB.Entities 可以很容易地建立关系，而无需手动进行连接，除非您真的想这样做:-)

看看下面的代码，它演示了作者和书籍实体之间的一对多关系。图书实体对作者一无所知。但是您仍然可以通过提供书籍 ID、书籍 ID 数组甚至书籍的 IQueryable 来获得反向关系访问。 [免责声明：我是图书馆的作者]

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

namespace StackOverflow
{
    public class Program
    {
        public class Author : Entity
        {
            public string Name { get; set; }
            public Many<Book> Books { get; set; }

            public Author() => this.InitOneToMany(() => Books);
        }

        public class Book : Entity
        {
            public string Title { get; set; }
        }

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

            var book = new Book { Title = "The Power Of Now" };
            book.Save();

            var author = new Author { Name = "Eckhart Tolle" };
            author.Save();

            author.Books.Add(book);

            //build a query for finding all books that has Power in the title.
            var bookQuery = DB.Queryable<Book>()
                              .Where(b => b.Title.Contains("Power"));

            //find all the authors of books that has a title with Power in them
            var authors = author.Books
                                .ParentsQueryable<Author>(bookQuery);

            //get the result
            var result = authors.ToArray();

            //output the aggregation pipeline
            Console.WriteLine(authors.ToString());


            Console.ReadKey();
        }
    }
}