LINQ to SQL 外键约束的插入及获取主表标识列等问题

主表YearJobCenter,从表YearJobIndex

插入部分代码，3个按钮事件分别为单独的插入主表、从表，和级联插入中从表：

值得注意的是，我开始的代码犯了一个低级错误

从表获取主表标识列ID不能用jobindex.Id = jdc.YearJobCenters.Select(c => c.Id).Single();

表中并非1条数据，当然会抛异常sequence contains more than one element。

正确：jobindex.Id = jdc.YearJobCenters.Max(p => p.Id);

  protected void btnInsertJobindex_Click(object sender, EventArgs e)
        {                
            try
            {
                using (StreamWriter sw = new StreamWriter(HttpContext.Current.Server.MapPath("logIndex.txt"), false))
                {                   
                    sw.WriteLine(DateTime.Now);
                    sw.WriteLine("---job index---");
                    jdc.Log = sw;
                  
                    YearJobIndex jobindex = new YearJobIndex();
                    /*jobindex.Id = jdc.YearJobCenters.Select(c => c.Id).Single();will throw exception: sequence contains more than one element */
                    jobindex.Id = jdc.YearJobCenters.Max(p => p.Id);
                    ScriptManager.RegisterClientScriptBlock(this.Button1, this.Button1.GetType(), "asfasfxjs", string.Format("alert('Select for DB\\r\\nLast JobCenter ID :{0}');", jobindex.Id), true);
                    jobindex.SchoolId =int.Parse( txtSchoolId.Text);
                    jobindex.Uid = Guid.NewGuid();
                    jobindex.Status = 0;
                    jobindex.IsLocked = false;
                    jdc.YearJobIndexes.InsertOnSubmit(jobindex);
                    jdc.SubmitChanges();
                    BindIndexData();
                }               
            }
            catch (Exception ex)
            {
                Response.Write(ex.ToString());
            }
        }

        protected void btnInsertJobcenter_Click(object sender, EventArgs e)
        {


            try
            {
                using (StreamWriter sw = new StreamWriter(Server.MapPath("logCenter.txt"), false))
                {
                    sw.WriteLine(DateTime.Now);
                    sw.WriteLine("---job center---");
                    jdc.Log = sw;

                    YearJobCenter jobcenter = new YearJobCenter();
                    
                
                    jobcenter.SchoolId = int.Parse(txtSchoolId.Text);
                    jobcenter.JobNumber = txtJobNumber.Text;
                    jobcenter.BookTypeEstimatated = 1;
                    jobcenter.OnlineImageCounter = 1;
                    jobcenter.OfflineImageCounter = 0;

                    jdc.YearJobCenters.InsertOnSubmit(jobcenter);

                    jdc.SubmitChanges();

                    string alertText = string.Format("alert('Increase Successful! \\r\\nLast JobCenter ID :{0}');", jobcenter.Id);

                    ScriptManager.RegisterClientScriptBlock(this.Button1, this.Button1.GetType(), "asfasfxjs", alertText, true);
                    BindCenterData();

                }
               
            }
            catch (Exception ex)
            {
                Response.Write(ex.ToString());
            }
        }

        protected void btnInsertCascade(object sender, EventArgs e)
        {
            try
            {
                using (StreamWriter sw = new StreamWriter(Server.MapPath("logCenter.txt"), false))
                {
                    sw.WriteLine(DateTime.Now);
                    sw.WriteLine("---job center and job index---");
                    jdc.Log = sw;

                    using (TransactionScope scope = new TransactionScope())
                    {
                        YearJobCenter jobcenter = new YearJobCenter();
                        jobcenter.SchoolId = int.Parse(txtSchoolId.Text);
                        jobcenter.JobNumber = txtJobNumber.Text;
                        jobcenter.BookTypeEstimatated = 1;
                        jobcenter.OnlineImageCounter = 1;
                        jobcenter.OfflineImageCounter = 0;

                        YearJobIndex jobindex = new YearJobIndex();
                        jobindex.SchoolId = jobcenter.SchoolId;
                        jobindex.Uid = Guid.NewGuid();
                        jobindex.Status = 0;
                        jobindex.IsLocked = false;
                        jobcenter.YearJobIndexes=jobindex;

                        jdc.YearJobCenters.InsertOnSubmit(jobcenter);
                        jdc.SubmitChanges();
                        
                        /*
                        YearJobIndex jobindex = new YearJobIndex();
                        jobindex.Id = jobcenter.Id;              
                        jobindex.SchoolId = jobcenter.SchoolId;
                        jobindex.Uid = Guid.NewGuid();
                        jobindex.Status = 0;
                        jobindex.IsLocked = false;
                        jdc.YearJobIndexes.InsertOnSubmit(jobindex);
                        jdc.SubmitChanges();
                        */


                        string alertText = string.Format("alert('Insert Cascade Successful!');");
                        ScriptManager.RegisterClientScriptBlock(this.Button1, this.Button1.GetType(), "asfasfxjs", alertText, true);
                        this.GridView1.DataSource = jdc.YearJobCenters.OrderBy(p => p.Id);
                        this.GridView1.DataBind();
                        this.GridView2.DataSource = jdc.YearJobIndexes.OrderBy(p => p.Id);
                        this.GridView2.DataBind();
                    }
                    
                }
            }
            catch (Exception ex)
            {
                Response.Write(ex.ToString());
            }

        }