linq：更新/删除联结表有两个主键

Question

    public void Update (Rate a)
    {
           a = (from ee in DataContext.Rates
                where ee.users_Id == a.users_Id && ee.books_Id == a.books_Id
                        select ee).SingleOrDefault();

        DataContext.SubmitChanges();
    }

错误：

  

违反PRIMARY KEY约束'PK_Rates'。无法在对象'dbo.Rates'中插入重复键。重复键值为（3,1）。