Entity framework 4 删除/更新多对多实体框架。无法让它工作

我正在使用EF4，并且在项目的多对多更新和删除方面存在问题。 我很高兴插入，但更新和删除无法解决它

   ClassID-ClassName 

假设我有3个表和3个匹配的dto类

   ClassID-ClassName 

课桌

   ClassID-ClassName 

学生桌

   ClassID-ClassName 

   StudentID-StudentName

学生课表

   ClassID-ClassName 

   StudentID-ClassID


///Happy all works I am using existing students to populate the class. Fine.
private void InsertClassRoom(ClassRoomDto classRoomDto)
{
    using (var ctx = new TrainingContext())
    {
        //Convert dto to Entity
        var classRoomEntity = new ClassRoom { ClassID = classRoomDto.ClassId, ClassName = classRoomDto.ClassName };
        foreach (var studentInClass in classRoomDto.Students)
        {
            Student student = ctx.Students.Where(x => x.StudentID == studentInClass.StudentId).SingleOrDefault();
            classRoomEntity.Students.Add(student);
        }
        ctx.AddToClassRooms(classRoomEntity);
        ctx.SaveChanges();
    }
}

但我有两个场景无法确定该做什么

   ClassID-ClassName 

更新教室名称

增加1名学生

更新1名学生的姓名

从课堂上删除学生。
我该怎么做

   ClassID-ClassName 

这是我的更新尝试：

   ClassID-ClassName 

private void UpdateClassRoom(ClassRoomDto classRoomDto)
{
    using (var ctx = new TrainingContext())
    {
        var classRoomEntity = new ClassRoom { ClassID = classRoomDto.ClassId, ClassName = classRoomDto.ClassName };
        foreach (var studentDto in classRoomDto.Students)
        {
            if (studentDto.StudentId == 0)
            {
                //it's a new student add it to the classroom
                Student student = new Student { StudentID = studentDto.StudentId, StudentName = studentDto.StudentName };
                classRoomEntity.Students.Add(student);
            }
            else
            {
                //Alter name of the student
                Student student = ctx.Students.Where(x => x.StudentID == studentDto.StudentId).SingleOrDefault();
                //update name
                student.StudentName = studentDto.StudentName;
                //? what do I do finish this attach or ???
           }
        }
        ctx.AddToClassRooms(classRoomEntity);
        ctx.SaveChanges();
    }
}

public void DeleteStudent(ClassRoomDto classRoomDto)
{
    using (var ctx = new TrainingContext())
    {
        //lost on how to delete a student in  many to many 
    }
}

---

首先，我假设您知道数据库中已经存在教室。最简单的方法是先查询它。实际上，插入新的

课堂

条目的原因是您使用了

ctx.AddToClassRooms（classRoomEntry）

。这会将实体附加到上下文，并将

EntityState

设置为

Added

   ClassID-ClassName 

private void UpdateClassRoom(ClassRoomDto classRoomDto)
{
    using (var ctx = new TrainingContext())
    {
        ClassRoom classRoomEntity = ctx.
                                    ClassRooms.
                                    Include("Students").
                                    Single(c => c.ClassID == classRoomDto.ClassId);
        classRoomEntity.ClassName = classRoomDto.ClassName;

        foreach (var studentDto in classRoomDto.Students)
        {
            if (studentDto.StudentId == 0)
            {
                // it's a new student add it to the classroom
                Student student = new Student
                                      {
                                          StudentID = studentDto.StudentId,
                                          StudentName = studentDto.StudentName
                                      };
                classRoomEntity.Students.Add(student);
            }
            else
            {
                // Student exists in the DB, but you don't know whether it's 
                // already part of the student collection for the classroom
                Student student = classRoomEntity.
                                  Students.
                                  FirstOrDefault(s => s.StudentID == studentDto.StudentId);

                if (student == null)
                {
                    // this student is not in the class, fetch it from the DB
                    // and add to the classroom
                    student = ctx.
                              Students.
                              SingleOrDefault(s => s.StudentID == studentDto.StudentId)

                    classRoomEntity.Students.Add(student);
                }

                // Update name
                student.StudentName = studentDto.StudentName;
                // Since student is now part of the classroom student collection
                // and classroom IS attached => student is also attached
            }
        }

        ctx.SaveChanges();
    }
}

要从thr类中删除学生，只需将其从集合中删除（但不要调用

ctx.DeleteObject（））

，因为这将从数据库中删除该学生。在您的情况下，上面的代码不会处理此问题，因为它只会添加新学生。您可以采取更简单的方法，而不是匹配数据库中但不在DTO中的所有学生。先清除列表，然后添加学生：

   ClassID-ClassName 

private void UpdateClassRoom(ClassRoomDto classRoomDto)
{
    using (var ctx = new TrainingContext())
    {
        ClassRoom classRoomEntity = ctx.
                                    ClassRooms.
                                    Include("Students").
                                    Single(c => c.ClassID == classRoomDto.ClassId);
        classRoomEntity.ClassName = classRoomDto.ClassName;
        classRoomEntity.Students.Clear();

        foreach (var studentDto in classRoomDto.Students)
        {
            Student student;
            if (studentDto.StudentId == 0)
            {
                // it's a new student add it to the classroom
                student = new Student
                              {
                                  StudentID = studentDto.StudentId,
                                  StudentName = studentDto.StudentName
                              };
            }
            else
            {
                student = ctx.
                          Students.
                          SingleOrDefault(s => s.StudentID == studentDto.StudentId)

                // Update name
                student.StudentName = studentDto.StudentName;
            }

            classRoomEntity.Students.Add(student);
        }

        ctx.SaveChanges();
    }
}

---

这可能是您正在寻找的方法。我专门编写了第一个代码片段，向您展示如何处理新实体和现有实体的不同情况，但它是第二个（更简单）更正确的方法。希望这有帮助。

太棒了。非常感谢你的帮助！！我正在努力学习EF，我正在做一个真实的项目，你的帮助非常巨大。谢谢。

   ClassID-ClassName