Question

我使用的是EF4，并且在更新和删除项目时遇到问题。我对插入很满意，但更新和删除无法弄明白。

假设我有3个表和3个匹配的dto类

ClassRoom表
```
   ClassID-ClassName 
```
学生表
```
   StudentID-StudentName
```

StudentClass表

   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2个场景无法弄清楚该怎么做。

更新教室名称
添加1名学生
更新1名学生的姓名

从班级中删除学生我该怎么做？

这是我的更新尝试：

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 
    }
}

Answer 1

首先，我假设你知道教室已经存在于数据库中。最简单的方法是首先查询它。您实际插入新ClassRoom条目的原因是您使用ctx.AddToClassRooms(classRoomEntry)。这会将实体附加到上下文，并将EntityState设置为Added。

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();
    }
}

为了从thr课程中删除学生，您只需将其从集合中删除（但不要调用ctx.DeleteObject()，因为这会将学生从数据库中删除在你的情况下，上面的代码不会处理这个问题，因为它只会增加新的学生。而不是匹配数据库中的所有学生，而不是你的DTO中的学生，你可以采取一种更简单的方法。清除列表首先，然后添加学生：

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。我特意编写了第一个代码片段，向您展示了如何处理新实体和现有实体的不同情况，但这是第二种（更简单）方法更正确。希望这会有所帮助。

删除/更新多对多实体框架。无法让它发挥作用

1 个答案: