Entity Framework之DB First方式详解

EF(Entity Framework的简称，下同)有三种方式，分别是：DataBase First、 Model First和Code First。

下面是Db First的方式：

1. 数据库库中存在两个表，一个是专业表，一个学生表，一个学生只能属于一个专业：

其中T_Major是专业表，T_Student是学生表，StudentId是学号，MajorId是专业Id，T_Major与T_Student是一对多的关系。

2. 项目中添加数据库实体模型

因为之前没有配置过数据库连接，所以点击“新建库连接”，如果之前配置过数据库连接，可以直接从下拉列表中选择或者新建

选择需要生成的表/存储过程等

点击“完成”

这里会弹出如下图的窗口，然后选择确定(如果再弹出，也选择确定)，如果不小心点击了取消，可以在模型设计界面Ctrl + S(保存的快捷键)，或如下图的操作，然后会弹出窗口，一直确定就行。

这里是使用MVC，所以添加一个控制器来测试（这里为了快速生成读写的控制器方法，选择“包含读/写操作的MVC5控制器”）

生成代码如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace Zhong.Web.Controllers
{
  public class StudentController : Controller
  {
    // GET: Student
    public ActionResult Index()
    {
      return View();
    }

    // GET: Student/Details/5
    public ActionResult Details(int id)
    {
      return View();
    }

    // GET: Student/Create
    public ActionResult Create()
    {
      return View();
    }

    // POST: Student/Create
    [HttpPost]
    public ActionResult Create(FormCollection collection)
    {
      try
      {
        // TODO: Add insert logic here

        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }

    // GET: Student/Edit/5
    public ActionResult Edit(int id)
    {
      return View();
    }

    // POST: Student/Edit/5
    [HttpPost]
    public ActionResult Edit(int id, FormCollection collection)
    {
      try
      {
        // TODO: Add update logic here

        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }

    // GET: Student/Delete/5
    public ActionResult Delete(int id)
    {
      return View();
    }

    // POST: Student/Delete/5
    [HttpPost]
    public ActionResult Delete(int id, FormCollection collection)
    {
      try
      {
        // TODO: Add delete logic here

        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }
  }
}

同样的方法添加一个Major控制器

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace Zhong.Web.Controllers
{
  public class MajorController : Controller
  {
    // GET: Major
    public ActionResult Index()
    {
      return View();
    }

    // GET: Major/Details/5
    public ActionResult Details(int id)
    {
      return View();
    }

    // GET: Major/Create
    public ActionResult Create()
    {
      return View();
    }

    // POST: Major/Create
    [HttpPost]
    public ActionResult Create(FormCollection collection)
    {
      try
      {
        // TODO: Add insert logic here

        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }

    // GET: Major/Edit/5
    public ActionResult Edit(int id)
    {
      return View();
    }

    // POST: Major/Edit/5
    [HttpPost]
    public ActionResult Edit(int id, FormCollection collection)
    {
      try
      {
        // TODO: Add update logic here

        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }

    // GET: Major/Delete/5
    public ActionResult Delete(int id)
    {
      return View();
    }

    // POST: Major/Delete/5
    [HttpPost]
    public ActionResult Delete(int id, FormCollection collection)
    {
      try
      {
        // TODO: Add delete logic here

        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }
  }
}

由于学生表MajorId依赖于Major表，所以需要先有专业，才能新增学生数据(这里不讨论是否合理)

编写逻辑代码，创建视图

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Zhong.Web.Models;

namespace Zhong.Web.Controllers
{
  public class MajorController : Controller
  {
    // GET: Major
    public ActionResult Index()
    {
      var majors = new EFDbEntities().T_Major.ToList();
      return View(majors);
    }

    // GET: Major/Details/5
    public ActionResult Details(int id)
    {
      var major = new EFDbEntities().T_Major.Find(id);
      if (major == null)
      {
        return Content("参数错误");
      }
      return View(major);
    }

    // GET: Major/Create
    public ActionResult Create()
    {
      return View();
    }

    // POST: Major/Create
    [HttpPost]
    public ActionResult Create(T_Major entity)
    {
      if (entity != null)
      {
        var entities = new EFDbEntities();
        entities.T_Major.Add(entity);
        entities.SaveChanges();
      }
      return RedirectToAction("Index");
    }

    // GET: Major/Edit/5
    public ActionResult Edit(int id)
    {
      var entity = new EFDbEntities().T_Major.Find(id);
      if (entity == null)
      {
        return Content("参数错误");
      }
      return View(entity);
    }

    // POST: Major/Edit/5
    [HttpPost]
    public ActionResult Edit(T_Major entity)
    {
      if (entity == null)
      {
        return Content("参数错误");
      }
      var entities = new EFDbEntities();
      #region 方式一
      ////该方式一般是根据主键先读取数据，然后再逐个赋值，最后更新
      //var oldEntity = entities.T_Major.Find(entity.Id);
      //if (oldEntity!=null)
      //{
      //  oldEntity.Name = entity.Name;
      //  entities.SaveChanges();
      //}
      #endregion

      #region 方式二
      //该方式是直接将新的实体(可能是new出来的并且对主键等的属性赋值好了)附加到上下文，然后标记状态为修改Modified
      entities.T_Major.Attach(entity);
      entities.Entry(entity).State = System.Data.Entity.EntityState.Modified;
      entities.SaveChanges();
      #endregion
      return RedirectToAction("Index");
    }

    // GET: Major/Delete/5
    public ActionResult Delete(int id)
    {
      var major = new EFDbEntities().T_Major.Find(id);
      return View(major);
    }

    // POST: Major/Delete/5
    [HttpPost]
    public ActionResult Delete(int id, FormCollection collection)
    {
      try
      {
        // TODO: Add delete logic here
        var entities = new EFDbEntities();
        var major = entities.T_Major.Find(id);
        entities.T_Major.Remove(major);
        entities.SaveChanges();
        return RedirectToAction("Index");
      }
      catch
      {
        return View();
      }
    }
  }
}

添加专业：

专业列表：

同样实现学生控制器与视图：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Zhong.Web.Models;

namespace Zhong.Web.Controllers
{
  public class StudentController : Controller
  {
    private EFDbEntities entities = new EFDbEntities();
    // GET: Student
    public ActionResult Index()
    {
      var students = entities.T_Student.ToList();
      return View(students);
    }

    // GET: Student/Details/5
    public ActionResult Details(int id)
    {
      var student = entities.T_Student.Find(id);
      return View(student);
    }

    // GET: Student/Create
    public ActionResult Create()
    {
      ViewData["MajorId"] = entities.T_Major.Select(m => new SelectListItem { Text = m.Name, Value = m.Id.ToString() });
      return View();
    }

    // POST: Student/Create
    [HttpPost]
    public ActionResult Create(T_Student entity)
    {
      entities.T_Student.Add(entity);
      entities.SaveChanges();
      return RedirectToAction("Index");
    }

    // GET: Student/Edit/5
    public ActionResult Edit(int id)
    {
      var student = entities.T_Student.Find(id);
      ViewData["MajorId"] = entities.T_Major.Select(m => new SelectListItem { Text = m.Name, Value = m.Id.ToString() });
      return View(student);
    }

    // POST: Student/Edit/5
    [HttpPost]
    public ActionResult Edit(T_Student entity)
    {
      if (entity == null)
      {
        return Content("参数错误");
      }
      entities.T_Student.Attach(entity);
      entities.Entry(entity).State = System.Data.Entity.EntityState.Modified;
      entities.SaveChanges();
      return RedirectToAction("Index");
    }

    // GET: Student/Delete/5
    public ActionResult Delete(int id)
    {
      var student = entities.T_Student.Find(id);
      return View(student);
    }

    // POST: Student/Delete/5
    [HttpPost]
    public ActionResult Delete(int id, FormCollection collection)
    {
      var student = entities.T_Student.Find(id);
      entities.T_Student.Remove(student);
      entities.SaveChanges();
      return RedirectToAction("Index");
    }
  }
}

添加学生时，报错如下：

于是在控制器中增加如下代码：

刷新页面：

编辑：

删除：

列表：

在MajorController中有介绍EF的两种更新方式。

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