Asp.net mvc 实体框架多对多关系添加了；完"；添加关系之前的项目

我已经建立了一个非常简单的实体框架数据库第一个项目，使用普通学生和课程场景

• 一个学生可以有很多课程
• 一门课程可以有很多学生

我已经在数据库中建立了关系，EF为此创建了一个很好的多对多导航项

问题当我向学生添加课程时，课程会重新添加到课程表中，然后关系会添加到数据库中，导致重复课程的数量不断增加

这让我快发疯了，知道我做错了什么吗

我的一些代码

我在实体生成的基础上创建了一个分部类，这样我就可以有一个带有复选框的EditorTemplate，这将允许用户勾选要添加的课程

public partial class Course
{
    public bool isSelected { get; set; }
}

public partial class Course
{
    public Course()
    {
        this.Students = new HashSet<Student>();
    }

    public int CourseId { get; set; }
    public string CourseName { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}

如果您能为我指出正确的方向，我们将不胜感激。

在将学生添加到上下文之前，您必须在您的post操作中将所选课程附加到上下文中。否则，EF假设整个对象图（包括所有课程的学生）是新的，必须插入到数据库中。通过附加（=将课程置于<代码>未更改状态），您告诉EF您不希望附加课程作为新行插入，而只希望在新学员和现有课程之间创建关系：

[HttpPost]
public ActionResult Create(CreateStudentViewModel model)
{
    if (ModelState.IsValid)
    {
        model.student.Courses =
            model.courses.Where(m => m.isSelected == true).ToList();

        foreach (var course in model.student.Courses)
            db.Courses.Attach(course);
        // now each course is in state Unchanged in the context

        db.Students.Add(model.student);
        // model.student is in state Added in the context
        // but the courses still have the state Unchanged.
        // Without the attach-loop above they would be in
        // state Added as well, which will create new courses
        // in the DB after SaveChanges

        db.SaveChanges();

        return RedirectToAction("Index");  
    }
    return View(model);
}

---

听起来很有道理，我会在周一试一试！非常感谢。

@model University.Models.CreateStudentViewModel            
@using (Html.BeginForm()) {
    @Html.ValidationSummary(true)
    <fieldset>
        <legend>Student</legend>            
        <div class="editor-label">
            @Html.LabelFor(model => model.student.Name)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.student.Name)
            @Html.ValidationMessageFor(model => model.student.Name)
        </div>    
        <div>        
            @Html.EditorFor(m=>m.courses)
        </div>
        <p>
            <input type="submit" value="Create" />
        </p>
    </fieldset>
}

public class CreateStudentViewModel
{
    public Student student { get; set; }
    public ICollection<Course> courses { get; set; }
}

public ActionResult Create()
{
    CreateStudentViewModel vm = new CreateStudentViewModel();
    vm.courses = db.Courses.ToList();
    return View(vm);
} 
[HttpPost]
public ActionResult Create(CreateStudentViewModel model)
{
    if (ModelState.IsValid)
    {
        model.student.Courses = model.courses.Where(m => m.isSelected == true).ToList();
        db.Students.Add(model.student);
        db.SaveChanges();
        return RedirectToAction("Index");  
    }
    return View(model);
}

[HttpPost]
public ActionResult Create(CreateStudentViewModel model)
{
    if (ModelState.IsValid)
    {
        model.student.Courses =
            model.courses.Where(m => m.isSelected == true).ToList();

        foreach (var course in model.student.Courses)
            db.Courses.Attach(course);
        // now each course is in state Unchanged in the context

        db.Students.Add(model.student);
        // model.student is in state Added in the context
        // but the courses still have the state Unchanged.
        // Without the attach-loop above they would be in
        // state Added as well, which will create new courses
        // in the DB after SaveChanges

        db.SaveChanges();

        return RedirectToAction("Index");  
    }
    return View(model);
}