c# - 如何在 C# Mongodb 强类型驱动程序中基于嵌套数组元素创建索引

Question

这个问题和this one的原则是一样的但是当此对象嵌套在集合的数组中时，我想在对象属性上使用强类型方法创建索引。

我可以使用:

new CreateIndexModel<T>( Builders<T>.IndexKeys.Ascending( a ) )

其中 a 是一个访问直接属性的表达式。

但我没有发现类似的内容:

Builders<Library>.Filter.ElemMatch(x => x.Author.Books, b => b.IsVerified == false));

这样我就可以将嵌套在作为集合成员的数组中的对象的某些字段定义为索引。

这有可能吗？如何做？

Answer 1

考虑以下数据模型:

public class Course
{
   public string Name { get; set; }
   public string Teacher { get; set; }
}

public class Student
{
  public string Name { get; set; }
  public int Age { get; set; }
  public ReadOnlyCollection<Course> Courses { get; set; }
}

您可以按以下方式在字段 Courses 上创建升序多键索引:

using MongoDB.Driver;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace ConsoleApp1
{
  public static class Program
  {
    private static MongoClient Client;
    private static IMongoDatabase Database;
    private static IMongoCollection<Student> Collection;

    public static async Task Main(string[] args)
    {
      Client = new MongoClient();
      Database = Client.GetDatabase("test-index");
      Collection = Database.GetCollection<Student>("students");

      var courses1 = new List<Course>()
      {
        new Course { Name = "Math", Teacher = "Bob" }
      }.AsReadOnly();

      var courses2 = new List<Course>()
      {
        new Course { Name = "Computer Science", Teacher = "Alice" }
      }.AsReadOnly();

      var mark = new Student
      {
        Name = "Mark",
        Courses = courses1,
        Age = 20
      };

      var lucas = new Student
      {
        Name = "Lucas",
        Courses = courses2,
        Age = 22
      };

      await Collection.InsertManyAsync(new[] { mark, lucas }).ConfigureAwait(false);


      var model = new CreateIndexModel<Student>(
        Builders<Student>.IndexKeys.Ascending(s => s.Courses));

      await Collection.Indexes.CreateOneAsync(model).ConfigureAwait(false);

      Console.WriteLine("All done !");
    }
  }
}

此查询由您创建的索引提供服务: db.students.find({Courses: {"Name": "Math", "Teacher": "Bob"}})

如果您不想在整个数组 Courses 上创建索引，而是想在嵌套对象(Course 对象)的字段 Name 上创建索引，这是方法:

using MongoDB.Driver;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace ConsoleApp1
{
  public static class Program
  {
    private static MongoClient Client;
    private static IMongoDatabase Database;
    private static IMongoCollection<Student> Collection;

    public static async Task Main(string[] args)
    {
      Client = new MongoClient();
      Database = Client.GetDatabase("test-index");
      Collection = Database.GetCollection<Student>("students");

      var courses1 = new List<Course>()
      {
        new Course { Name = "Math", Teacher = "Bob" }
      }.AsReadOnly();

      var courses2 = new List<Course>()
      {
        new Course { Name = "Computer Science", Teacher = "Alice" }
      }.AsReadOnly();

      var mark = new Student
      {
        Name = "Mark",
        Courses = courses1,
        Age = 20
      };

      var lucas = new Student
      {
        Name = "Lucas",
        Courses = courses2,
        Age = 22
      };

      await Collection.InsertManyAsync(new[] { mark, lucas }).ConfigureAwait(false);


      var model = new CreateIndexModel<Student>(
        Builders<Student>.IndexKeys.Ascending("Courses.Name"));

      await Collection.Indexes.CreateOneAsync(model).ConfigureAwait(false);

      Console.WriteLine("All done !");
    }
  }
}

此查询由您创建的索引提供服务:db.students.explain("executionStats").find({"Courses.Name": "Math"})

在我的第二个示例中避免使用魔术字符串的一种可能方法是利用 nameof C# 运算符的强大功能: $"{nameof(Student.Courses)}.{nameof(Course.Name)}"