Entity Framework之DB First方式

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