使用DbContext中的同一个键保留多个对象

Question

我正在尝试使用实体框架代码优先来创建ASP.NET MVC Web应用程序。 我有两个简单的模型

public class Assignment 
    { 
     public int Id { get; set; } 
     [ForeignKey("Initiator")] 
     public int InititatorId { get; set; } 
     public virtual User Initiator { get; set; } 

     public virtual List<User> Debtors { get; set; } 
     public virtual List<User> Responsibles { get; set; } 

     public Assignment() 
     { 
      Debtors = new List<User>(); 
      Responsibles = new List<User>(); 
     } 
    } 

public class User 
    { 
     public int Id { get; set; } 
     public string Name { get; set; } 
     public string City { get; set; } 
    } 

和我的DbContext看起来像

public class AssignmentContext : DbContext 
    { 

     public DbSet<Assignment> Assignments { get; set; } 
     public DbSet<User> Users { get; set; } 

     protected override void OnModelCreating(DbModelBuilder modelBuilder) 
     { 
      modelBuilder.Entity<Assignment>() 
       .HasRequired(r => r.Initiator) 
       .WithMany() 
       .WillCascadeOnDelete(false); 

      modelBuilder.Entity<Assignment>() 
       .HasMany(m => m.Responsibles) 
       .WithMany() 
       .Map(m => 
       { 
        m.MapLeftKey("AssignmentId"); 
        m.MapRightKey("UserId(Responsible)"); 
        m.ToTable("AssignmentResponsibles"); 
       }); 

      modelBuilder.Entity<Assignment>() 
       .HasMany(m => m.Debtors) 
       .WithMany() 
       .Map(m => 
        { 
         m.MapLeftKey("AssignmentId"); 
         m.MapRightKey("UserId(Debtor)"); 
         m.ToTable("AssignmentDebtors"); 
        }); 
     } 

有没有办法保持相同的用户实例在列表债务人和Responsibles？

因为我在尝试保存更改时遇到错误。

An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key. 

我需要这样做，因为它是应用程序逻辑所要求的。

在此先感谢。

更新：

上。这里HttpPost错误出现（控制台应用样品中的相同的错误）：从视图

[HttpPost] 
     [ValidateAntiForgeryToken] 
     public ActionResult Edit([Bind(Include = "Id,Text,CreationDate,InititatorId,Debtors,Responsibles")] Assignment assignment) 
     { 
      if (ModelState.IsValid) 
      { 
       assignment.Debtors.RemoveAll(r => r.Name == "Roman"); 
       db.Entry(assignment).State = EntityState.Modified; // HERE! 
       db.SaveChanges(); 
       return RedirectToAction("Index"); 
      } 
      ViewBag.InititatorId = new SelectList(db.Users, "Id", "Name", assignment.InititatorId); 
      return View(assignment); 
     } 

借方和Responsbiles元件由隐藏的输入通过。

Answer 1

为什么不先直接获取数据，而是直接将其附加到dbcontext中，具有相同标识的相同分配可能已经在同一个上下文中本地连接。

，而不是这些

assignment.Debtors.RemoveAll(r => r.Name == "Roman"); 
db.Entry(assignment).State = EntityState.Modified; // HERE! 
db.SaveChanges(); 

尝试这些

var assignmentDb = db.Assignments.FirstOrDefault(asg => asg.Id == assignment.Id); 
assignmentDb.Debtors.RemoveAll(r => r.Name == "Roman"); 
db.SaveChanges();