查询多对多用户->类

问题描述

我有两个实体，用户和类，具有多对多关系。我正在使用.Net Core，并且在此博客文章HERE（直到第3部分）之后建立了关系。数据库可以工作，并且我能够添加类并填充联接表，但是我不知道如何检索用户已注册的所有类，或者以其他方式检索已注册特定用户的所有用户课。

我需要解决用户已经登录到类而不必在连接表中添加新条目的情况（这样仍然会引发错误）。

User.cs

public class User : IdentityUser
{
    // Making the classes a JoinCollectionFacade allows us to perform actions like adding a class
    public User()
        => Classes = new JoinCollectionFacade<Class,UserClass>(
            UserClass,uc => uc.Class,c => new UserClass { User = this,Class = c });

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Type { get; set; }
    //public string Email { get; set; }
    public string Phone { get; set; }
    public string Membership { get; set; }

    private ICollection<UserClass> UserClass { get; } = new List<UserClass>();

    // This element does not need to be mapped. It's used for easier reference
    // to the Users enrolled in the class
    [NotMapped]
    public ICollection<Class> Classes { get; }
}

Class.cs

public class Class
{

    // Making the classes a JoinCollectionFacade allows us to perform actions like adding a class
    public Class()
        => Users = new JoinCollectionFacade<User,uc => uc.User,u => new Models.UserClass { User = u,Class = this });

    public Class(Instructor i,string d)
    {
        Instructor = i;
        Description = d;
    }

    public int Id { get; set; }
    public virtual Instructor Instructor { get; set; } // Made virtual so we can take advantage of certain Entity Framework such as lazy loading (from Microsoft docs)
    public string Description { get; set; }
    public DateTime StartTime { get; set; }
    public DateTime EndTime { get; set; }

    private ICollection<UserClass> UserClass { get; } = new List<UserClass>();

    // This element does not need to be mapped. It's used for easier reference
    // to the Users enrolled in the class
    [NotMapped]
    public ICollection<User> Users { get; }
}

UserClass.cs

public class UserClass
{
    public string UserId { get; set; }
    public User User { get; set; }

    public int ClassId { get; set; }
    public Class Class { get; set; }
}

解决方法

我通过以下方式解决了我的问题

• 在User.cs和Class.cs中公开UserClass实例
• 在下面使用这种查询来检索所有注册特定班级的用户

var usersInClass = _context.Users
                      .Include(i => i.UserClass).ThenInclude(i => i.Class)
                      .Where(x => x.UserClass.Select(pc => pc.Class).Any(c => c.Id == id))
                      .ToList();