MVC CodeFirst计算属性获取错误”；ObjectContext实例已释放”；

我有一个ASP。NET MVC Code First网站。我有一个模型Teacher，它有一个Student的列表，每个列表都有一个计算的属性CompletedTests。当我试图从包含Student s列表的ViewModel中访问计算属性CompletedTests时，出现以下错误：

An exception of type 'System.ObjectDisposedException' occurred in EntityFramework.dll but was not handled in user code
Additional information: The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.  

教师：

public class Teacher
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Student> Students{ get; set; }
}

学生：

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int TeacherId { get; set; }
    public virtual Teacher Teacher { get; set; }
    public int TotalTests
    {
        get
        {
            return Tests.Count;
        }
    }
    public int CompletedTests
    {
        get
        {
            return Tests.Count(p => p.IsCompleted);
        }
    }
    public virtual ICollection<Test> Tests{ get; set; }
}

测试：

public class Test
{
    public int Id { get; set; }
    public int StudentId { get; set; }
    public int QuestionsTotal
    {
        get
        {
            return Questions.Count;
        }
    }
    public bool IsCompleted
    {
        get
        {
            return Questions.Count(q => q.Completed) >= QuestionsTotal;
        }
    }
    public virtual Student Student { get; set; }
    public virtual ICollection<Question> Questions{ get; set; }
}

问题：

public class Question
{
    public int Id { get; set; }
    public int TestId { get; set; }
    public virtual Question Question { get; set; }
}

HomeViewModel：

public class HomeViewModel
{
    public string TeacherName { get; set; }
    public int StudentCount { get; set; }
    public IEnumerable<Student> Students { get; set; }
}

家庭控制器：

public ActionResult Index()
{
    var model = new HomeViewModel();
    var userId = User.Identity.GetUserId();
    using (var context = new ApplicationDbContext())
    {
        var teacher = context.Teachers.FirstOrDefault(c => c.IdentityId == userId);
        if (teacher != null)
        {
            model.TeacherName = teacher.Name;
            model.Students = teacher.Students.ToList();
        }
        return View(model);
    }
}

Index.chtml部分：

        <tbody>
            @foreach (var student in Model.Students)
            {
                <tr>
                    <td>
                        @student.Name
                    </td>
                    <td>
                        @(student.CompletedTests + "/" + student.TotalTests)
                    </td>
                </tr>
            }
        </tbody>

有人能指出为什么我在student.CompletedTests部分得到已处理的实例错误吗？

model.Students = teacher.Students.Include(t => t.Tests).ToList();