ASP.NET Core 6/EF Core双向关联异常：User集合属性始终为空

定义了User和Friendship两个模型，Friendship中关联RequestUser或ReceiveUser时能获取正确数据，但User中的RequestedFriendships和ReceivedFriendships集合始终为空（数据库中存在对应关联数据），求解决办法。

模型代码 #

public class User
{
    public int Id { get; set; }
    public string? Email { get; set; }
    public string? Password { get; set; }

    public virtual ICollection<Friendship> RequestedFriendships { get; set; }
    public virtual ICollection<Friendship> ReceivedFriendships { get; set; }
}

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

    public int? RequestUserId { get; set; }
    public int? ReceiveUserId { get; set; }

    public FriendshipStatus Status { get; set; }
    public DateTime CreationDate { get; set; }
    public DateTime? UpdatedDate { get; set; }
    public virtual User RequestUser { get; set; }
    public virtual User ReceiveUser { get; set; }
}

Fluent API配置代码 #

modelBuilder.Entity<Friendship>()
            .HasOne(f => f.RequestUser)
            .WithMany(u => u.RequestedFriendships)
            .HasForeignKey(f => f.RequestUserId)
            .OnDelete(DeleteBehavior.ClientSetNull);

modelBuilder.Entity<Friendship>()
            .HasOne(f => f.ReceiveUser)
            .WithMany(u => u.ReceivedFriendships)
            .HasForeignKey(f => f.ReceiveUserId)
            .OnDelete(DeleteBehavior.ClientSetNull);

• 启用延迟加载或显式预加载关联集合
  你的导航属性已标记为virtual，但EF Core默认不启用延迟加载代理。可在DbContext的OnConfiguring方法中开启：

  protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
  {
      optionsBuilder.UseLazyLoadingProxies()
                    .UseSqlServer("你的数据库连接字符串");
  }
  

  若不想使用延迟加载，查询User时必须显式通过Include预加载关联集合：

  var targetUser = dbContext.Users
      .Include(u => u.RequestedFriendships)
      .Include(u => u.ReceivedFriendships)
      .FirstOrDefault(u => u.Id == 目标用户ID);
  }
  

• 验证数据库外键与数据有效性
  确认Friendship表的RequestUserId和ReceiveUserId外键正确关联到User表的Id字段，同时检查数据库中这些外键值是否对应存在的User记录，避免无效ID导致关联失效。

• 确认DbContext已注册实体
  确保DbContext中声明了对应的DbSet：

  public class YourDbContext : DbContext
  {
      public DbSet<User> Users { get; set; }
      public DbSet<Friendship> Friendships { get; set; }
  
      // 其他配置代码
  }
  

内容的提问来源于stack exchange，提问作者Mortimen