如何通过Entity Framework同时向3张关联表提交数据？

看起来你已经搭好了基础的视图和控制器逻辑，现在只需要完善视图的工作日选择，以及编写POST提交的逻辑来关联三个表的数据。下面一步步来帮你搞定：

1. 修正视图绑定并添加工作日选择项 #

你的视图目前缺少WeekDay的选择模块，而且Teacher的绑定逻辑有点小问题（应该直接绑定到TeacherId字段，而不是Teacher.Id）。更新后的Create视图代码如下：

<div class="form-horizontal">
    <h4>Appointment</h4>
    <hr />
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(model => model.TeacherId, "Teacher Name", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            <!-- 修正绑定目标为Appointment.TeacherId字段 -->
            @Html.DropDownListFor(m => m.TeacherId, (List<SelectListItem>)ViewBag.TeacherId, "Select a teacher", new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.TeacherId, "", new { @class = "text-danger" })
        </div>
    </div>
    <!-- 新增工作日选择下拉框 -->
    <div class="form-group">
        @Html.Label("WeekDayId", "Week Day", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownList("WeekDayId", (List<SelectListItem>)ViewBag.DayId, "Select a weekday", new { @class = "form-control" })
            @Html.ValidationMessage("WeekDayId", "", new { @class = "text-danger" })
        </div>
    </div>
    <div class="form-group">
        @Html.LabelFor(model => model.Hours,"Working Hour", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.Hours, new { htmlAttributes = new { @class = "form-control", placeholder = "e.g. 09:00 - 17:00" } })
            @Html.ValidationMessageFor(model => model.Hours, "", new { @class = "text-danger" })
        </div>
    </div>
    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" value="Create" class="btn btn-default" />
        </div>
    </div>
</div>

这里的核心调整：

• 将教师下拉框直接绑定到Appointment.TeacherId，让模型绑定能直接获取选中的教师ID
• 添加工作日选择模块，用WeekDayId作为表单字段名，后续在POST方法中单独接收

2. 编写POST版Create方法处理数据提交 #

现在需要实现提交逻辑，接收表单数据后关联Teacher和WeekDay，最终保存到数据库。注意因为Appointment和WeekDay是多对多关系，我们需要从数据库中获取已存在的实体来建立关联，避免EF误创建新的工作日/教师记录：

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(Appointment appointment, int WeekDayId)
{
    if (ModelState.IsValid)
    {
        // 1. 获取数据库中已存在的教师实体，建立关联
        var targetTeacher = db.Teachers.Find(appointment.TeacherId);
        appointment.Teacher = targetTeacher;

        // 2. 获取选中的工作日实体，添加到Appointment的关联集合中
        var targetWeekDay = db.WeekDays.Find(WeekDayId);
        appointment.WeekDays.Add(targetWeekDay);

        // 3. 将Appointment存入数据库并保存所有变更
        db.Appointments.Add(appointment);
        db.SaveChanges();

        return RedirectToAction("Index");
    }

    // 验证失败时，重新填充下拉列表并返回视图
    var myTeacherList = (from teacher in db.Teachers.ToList()
                        select new SelectListItem {
                            Text = teacher.Firstname + " " + teacher.Lastname, // 增加空格优化显示
                            Value = teacher.Id.ToString(),
                        }).ToList();
    var myDayNameList = (from day in db.WeekDays.ToList()
                        select new SelectListItem {
                            Text = day.Name,
                            Value = day.Id.ToString(),
                        }).ToList();
    ViewBag.TeacherId = myTeacherList;
    ViewBag.DayId = myDayNameList;

    return View(appointment);
}

关键逻辑说明： #

• 通过WeekDayId参数接收选中的工作日ID（因为Appointment实体没有直接的WeekDayId字段，所以单独作为参数接收）
• 使用Find()方法获取数据库中已存在的实体，确保EF只建立关联，不会重复插入教师/工作日数据
• 将获取到的WeekDay添加到Appointment.WeekDays集合，EF会自动维护多对多的中间表记录

3. 可选：添加模型验证增强用户体验 #

为了确保输入合法性，可以在Appointment实体的属性上添加验证特性：

public partial class Appointment {
    // ... 原有代码 ...
    [Required(ErrorMessage = "Please select a teacher")]
    public int TeacherId { get; set; }

    [Required(ErrorMessage = "Please enter working hours")]
    [Display(Name = "Working Hours")]
    public string Hours { get; set; }
    // ... 原有代码 ...
}

这样用户未选择教师或未输入工作时长时，页面会显示对应的错误提示。

测试你的示例场景 #

按照你给出的例子：

• 选择John Reese + 周五 + 09:00-17:00提交后，EF会：
  1. 创建一条Appointment记录，关联John的TeacherId，存入输入的工作时长
  2. 在Appointment与WeekDay的中间表中添加关联记录，绑定这条新Appointment和周五的WeekDay
• Harold Finch + 周一 + 11:00-15:00的记录也会被正确创建和关联

内容的提问来源于stack exchange，提问作者agahpars