asp.net/wingtip/创建数据访问层

一. 什么是数据访问层

在wingtip项目中，数据访问层是对以下三者的总称：
1. product类等数据相关的实体类(class)
2. 数据库(database)，对实体类成员的存储
3. 上述二者的交互操作。

product类：

 using System.ComponentModel.DataAnnotations;

 namespace WingtipToys.Models
 {
   public class Product
   {
     [ScaffoldColumn(false)]
     public int ProductID { get; set; }

     [Required, StringLength(), Display(Name = "Name")]
     public string ProductName { get; set; }

     [Required, StringLength(), Display(Name = "Product Description"), DataType(DataType.MultilineText)]
     public string Description { get; set; }

     public string ImagePath { get; set; }

     [Display(Name = "Price")]
     public double? UnitPrice { get; set; }

     public int? CategoryID { get; set; }

     public virtual Category Category { get; set; }
   }
 }

database(T-SQL)：

 CREATE TABLE [dbo].[Products] (
     [ProductID]   INT            IDENTITY (1, 1) NOT NULL,
     [ProductName] NVARCHAR (100) NOT NULL,
     [Description] NVARCHAR (MAX) NOT NULL,
     [ImagePath]   NVARCHAR (MAX) NULL,
     [UnitPrice]   FLOAT (53)     NULL,
     [CategoryID]  INT            NULL,
     CONSTRAINT [PK_dbo.Products] PRIMARY KEY CLUSTERED ([ProductID] ASC),
     CONSTRAINT [FK_dbo.Products_dbo.Categories_CategoryID] FOREIGN KEY ([CategoryID]) REFERENCES [dbo].[Categories] ([CategoryID])
 );

 GO
 CREATE NONCLUSTERED INDEX [IX_CategoryID]
     ON [dbo].[Products]([CategoryID] ASC);

---

二. 让实体类与数据库开始交流

创建好了product类以及对应的数据库，我们接下来要让将二者使用起来。首先是上下文类(context class)，这个类负责管理实体类(如product类、category类)，并且提供对数据库的访问操作。

productContext类：

 using System.Data.Entity;
 namespace WingtipToys.Models
 {
   public class ProductContext : DbContext
   {
     public ProductContext()
       : base("WingtipToys")
     {
     }

     public DbSet<Category> Categories { get; set; }
     public DbSet<Product> Products { get; set; }
     public DbSet<CartItem> ShoppingCartItems { get; set; }
     public DbSet<Order> Orders { get; set; }
     public DbSet<OrderDetail> OrderDetails { get; set; }

   }
 }

在程序第一次运行的时候，数据库虽然已经建立好了数据表，但这是一个空表，如何给这个数据表添加一些数据呢？建立一个ProductDatabaseInitializer类

ProductDatabaseInitializer类：

 using System.Collections.Generic;
 using System.Data.Entity;

 namespace WingtipToys.Models
 {
   public class ProductDatabaseInitializer : DropCreateDatabaseIfModelChanges<ProductContext>
   {
     protected override void Seed(ProductContext context)
     {
       GetCategories().ForEach(c => context.Categories.Add(c));
       GetProducts().ForEach(p => context.Products.Add(p));
     }

     private static List<Category> GetCategories()
     {
       var categories = new List<Category> {
                 new Category
                 {
                     CategoryID = ,
                     CategoryName = "Cars"
                 },
                 new Category
                 {
                     CategoryID = ,
                     CategoryName = "Planes"
                 },
                 new Category
                 {
                     CategoryID = ,
                     CategoryName = "Trucks"
                 },
                 new Category
                 {
                     CategoryID = ,
                     CategoryName = "Boats"
                 },
                 new Category
                 {
                     CategoryID = ,
                     CategoryName = "Rockets"
                 },
             };

       return categories;
     }

     private static List<Product> GetProducts()
     {
       var products = new List<Product> {
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Convertible Car",
                     Description = "This convertible car is fast! The engine is powered by a neutrino based battery (not included)." +
                                   "Power it up and let it go!",
                     ImagePath="carconvert.png",
                     UnitPrice = 22.50,
                     CategoryID =
                },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Old-time Car",
                     Description = "There's nothing old about this toy car, except it's looks. Compatible with other old toy cars.",
                     ImagePath="carearly.png",
                     UnitPrice = 15.95,
                      CategoryID =
                },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Fast Car",
                     Description = "Yes this car is fast, but it also floats in water.",
                     ImagePath="carfast.png",
                     UnitPrice = 32.99,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Super Fast Car",
                     Description = "Use this super fast car to entertain guests. Lights and doors work!",
                     ImagePath="carfaster.png",
                     UnitPrice = 8.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Old Style Racer",
                     Description = "This old style racer can fly (with user assistance). Gravity controls flight duration." +
                                   "No batteries required.",
                     ImagePath="carracer.png",
                     UnitPrice = 34.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Ace Plane",
                     Description = "Authentic airplane toy. Features realistic color and details.",
                     ImagePath="planeace.png",
                     UnitPrice = 95.00,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Glider",
                     Description = "This fun glider is made from real balsa wood. Some assembly required.",
                     ImagePath="planeglider.png",
                     UnitPrice = 4.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Paper Plane",
                     Description = "This paper plane is like no other paper plane. Some folding required.",
                     ImagePath="planepaper.png",
                     UnitPrice = 2.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Propeller Plane",
                     Description = "Rubber band powered plane features two wheels.",
                     ImagePath="planeprop.png",
                     UnitPrice = 32.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Early Truck",
                     Description = "This toy truck has a real gas powered engine. Requires regular tune ups.",
                     ImagePath="truckearly.png",
                     UnitPrice = 15.00,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Fire Truck",
                     Description = "You will have endless fun with this one quarter sized fire truck.",
                     ImagePath="truckfire.png",
                     UnitPrice = 26.00,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Big Truck",
                     Description = "This fun toy truck can be used to tow other trucks that are not as big.",
                     ImagePath="truckbig.png",
                     UnitPrice = 29.00,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Big Ship",
                     Description = "Is it a boat or a ship. Let this floating vehicle decide by using its " +
                                   "artifically intelligent computer brain!",
                     ImagePath="boatbig.png",
                     UnitPrice = 95.00,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Paper Boat",
                     Description = "Floating fun for all! This toy boat can be assembled in seconds. Floats for minutes!" +
                                   "Some folding required.",
                     ImagePath="boatpaper.png",
                     UnitPrice = 4.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Sail Boat",
                     Description = "Put this fun toy sail boat in the water and let it go!",
                     ImagePath="boatsail.png",
                     UnitPrice = 42.95,
                     CategoryID =
                 },
                 new Product
                 {
                     ProductID = ,
                     ProductName = "Rocket",
                     Description = "This fun rocket will travel up to a height of 200 feet.",
                     ImagePath="rocket.png",
                     UnitPrice = 122.95,
                     CategoryID =
                 }
             };

       return products;
     }
   }
 }

当数据库被创建并且初始化的时候，执行了该类的Seed()方法。这个方法在该类中，是对基类中Seed()方法的重写。执行结束后，数据库中就被填充了类别和产品两类数据。

---

三. 让web前台使用他们

在web应用启动的时候，为了能够使用我们前边创建好的各个类、数据库、操作逻辑等等，我们需要更新在Global.asax.cs中的Application_Start方法(handler)。

using WingtipToys.Models;
using WingtipToys.Logic;

namespace WingtipToys
{
    public class Global : HttpApplication
    {
        void Application_Start(object sender, EventArgs e)
        {
            // Code that runs on application startup
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);

            // Initialize the product database.(此处为方法的更新)
            Database.SetInitializer(new ProductDatabaseInitializer());

            // Create the custom role and user.
            RoleActions roleActions = new RoleActions();
            roleActions.AddUserAndRole();

            // Add Routes.
            RegisterCustomRoutes(RouteTable.Routes);
        }
       ...
       ...其他代码省略...
       ...
     }
}

在web应用启动时，在第一次访问数据的时候(product data)，程序会执行指定方法(initializer)来生成数据。

四. 总结

这一章就写这么多，主要是在数据和数据操作层面解释了什么是所谓的“数据访问层”。通过上述三个小节可以看到，“数据访问层”是对实体类(class)、数据库(database,table)、方法(method)的一个统称。对于实体类这样的.cs文件，我们将他们存放在Models文件夹下，也即给他们扣个帽子，“模型”指的就是他们。