如何使用Entity Framework在c＃中访问多对多映射表

Question

I am using EF code-first. I have two classes Users and Roles which are mapped in a many-to-many relationship as below.

public class User
{
    public User()
    {
        this.Roles = new HashSet<Role>();
    }

    public int UserId { get; set; }
    public string UserName { get; set; }
    public string PasswordHash { get; set; }
    public DateTime? LastLoggedOn { get; set; }
    public DateTime? PasswordChangedOn { get; set; }
    public int IsActive { get; set; }

    public virtual ICollection<Role> Roles { get; set; }
}

public class Role
{
    public Role()
    {
        this.Users = new HashSet<User>();
    }

    public int RoleId { get; set; }
    public string RoleName { get; set; }
    public string Description { get; set; }

    public virtual ICollection<User> Users { get; set; }
}

public class UserMap : EntityTypeConfiguration<User>
{
    public UserMap()
    {
        HasMany(r => r.Roles).WithMany(u=>u.Users)
            .Map(m =>
            {
                m.ToTable("UserRoles");
                m.MapLeftKey("UserId");
                m.MapRightKey("RoleId");
            });
    }
}

Now I want to fetch RoleName from Roles with respective to UserId from Users

I am using this code:

var roleName = db.Users.Join(db.UserRoles, u => u.UserId, r => r.UserId, (u, r) => new { u, r })
                       .Join(db.Roles, ur => ur.r.RoleId, q => q.RoleId, (ur, q) => new { ur, q })
                       .Where(m => m.ur.r.UserId == 1)
                       .Select(m => new { m.ur.r.UserId, m.q.RoleName });

But it does not allow me to use UserRoles .

Error

'Context' does not contain a definition for 'UserRoles' and no extension method 'UserRoles' accepting a first argument of type 'Context' could be found (are you missing a using directive or an assembly reference?)

Can anyone help me here?

Answer 1

The name of the table you define in the fluent API will only be used (and known) by the database. From your application, you should query it by doing :

db.Users.Find(userId).SelectMany(x=>x.Roles).SelectMany(x=>x.RoleName)

Note that this will load all Roles from the user in memory, but this is fine since you will never have 1000 roles for one user I guess.

Apart from that, I think that you should only have a 1 to 1 relationship between Roles and Users. I'd advise to add a UserId column in the role class and to remove the Users collection from this class.

This way, your query will simply be :

db.Roles.Where( x => x.UserId = userId).Select( x => x.RoleName);

And this time, only the Role Names will be pulled into memory. I think it is also a slightly better design.