EF Core 多对多关系无法为集合返回 null

Question

我想使用 MySQL 在 EF Core 中创建多对多关系，以便一个用户可以拥有多个角色，并且可以将角色分配给多个用户。我遵循了 Microsoft 的指南，但是当我运行

Users.SingleOrDefault()

时，结果的角色集合为空。

    public class User
    {
        public int Id { get; set; }

        [Required]
        public string FirstName { get; set; }

        [Required]
        public string LastName { get; set; }

        [Required]
        [EmailAddress]
        public string Email { get; set; }

        [Required]
        public string Password { get; set; }

        [Required]
        public ICollection<Role> Roles { get; set; }
    }

    public class Role
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public ICollection<User> Users { get; set; }
    }

    public class DatabaseService : DbContext
    {
        public DbSet<User> Users { get; set; }
        public DbSet<Role> Roles { get; set; }

        private static string connectionString = "Server=localhost;Database=mydb;Uid=user;Pwd=mypwd;";

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseMySql(connectionString, ServerVersion.AutoDetect(connectionString));
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<User>()
                .HasMany(u => u.Roles)
                .WithMany(u => u.Users)
                .UsingEntity("UsersRoles");
        }
    }

在数据库中保存和查询数据

Collection<Role> roles = new()
{
    new Role
    {
        Name = "Customer"
    },
    new Role
    {
        Name = "Administrator"
    }
};

User newUser = new()
{
    Email = register.Email,
    Password = hashedPassword,
    FirstName = register.FirstName,
    LastName = register.LastName,
    Roles = roles
};

database.Users.Add(newUser);
database.SaveChanges();

User? user = database.Users.SingleOrDefault(user => user.Email == login.Email && user.Password == HashPassword(login.Password));

Console.WriteLine(user.Roles); //null

尽管如此，数据库的连接表包含数据。

Answer 1

很抱歉，但我无法发表评论，这就是为什么我会发布答案：

尝试将用户设为虚拟，希望对您有帮助

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