Entity Framework之DB First方式详解

2020-01-17 22:19:09

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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：