entityframework学习笔记--007-实体数据建模基础之继承关系映射TPT

Table per Type Inheritance (TPT)建模

1.假设你有两张表与一张公共的表密切相关，如图7-1所示，Businiss表与eCommerce表、Retail表有1：0...1关系。最关键的是，eCommerce表和Retail表中有关于Business表中代表业务的额外的信息。

图7-1

2. 右键你的项目，新建三个实体类，其中eCommerce、Retail继承自Businiss。如下代码：

 [Table("Business", Schema = "example7")]
     public class Business
     {
         [Key]
         [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
         public int BusinessId { get; protected set; }
         public string Name { get; set; }
         public string LicenseNumber { get; set; }
     }

Business

 [Table("eCommerce", Schema = "example7")]
     public class eCommerce : Business
     {
         public string URL { get; set; }
     }

eCommerce

 [Table("Retail", Schema = "example7")]
     public class Retail : Business
     {
         public string Address { get; set; }
         public string City { get; set; }
         public string State { get; set; }
         public string ZIPCode { get; set; }
     }

Retail

3.创建一个继承自DbContext的上下文对象EF6RecipesContext；

 public class EF6RecipesContext : DbContext
     {
         public DbSet<Business> Businesses { get; set; }
         public EF6RecipesContext()
             : base("name=EF6CodeFirstRecipesContext")
         {
         }
         protected override void OnModelCreating(DbModelBuilder modelBuilder)
         {

         }
     }

EF6RecipesContext

4.测试代码：

 using (var context = new EF6RecipesContext())
             {
                 var business = new Business
                 {
                     Name = "Corner Dry Cleaning",
                     LicenseNumber = "100x1"
                 };
                 context.Businesses.Add(business);
                 var retail = new Retail
                 {
                     Name = "Shop and Save",
                     LicenseNumber = "200C",
                     Address = "101 Main",
                     City = "Anytown",
                     State = "TX",
                     ZIPCode = ""
                 };
                 context.Businesses.Add(retail);
                 var web = new eCommerce
                 {
                     Name = "BuyNow.com",
                     LicenseNumber = "300AB",
                     URL = "www.buynow.com"
                 };
                 context.Businesses.Add(web);
                 context.SaveChanges();
             }

             using (var context = new EF6RecipesContext())
             {
                 Console.WriteLine("\n--- All Businesses ---");
                 foreach (var b in context.Businesses)
                 {
                     Console.WriteLine("{0} (#{1}).{2}", b.Name, b.LicenseNumber,b.BusinessId);
                 }

                 Console.WriteLine("\n--- Retail Businesses ---");
                 foreach (var r in context.Businesses.OfType<Retail>())
                 {
                     Console.WriteLine("{0} (#{1}).{2}", r.Name, r.LicenseNumber,r.BusinessId);
                     Console.WriteLine("{0}", r.Address);
                     Console.WriteLine("{0}, {1} {2}", r.City, r.State, r.ZIPCode);
                 }

                 Console.WriteLine("\n--- eCommerce Businesses ---");
                 foreach (var e in context.Businesses.OfType<eCommerce>())
                 {
                     Console.WriteLine("{0} (#{1}).{2}", e.Name, e.LicenseNumber,e.BusinessId);
                     Console.WriteLine("Online address is: {0}", e.URL);
                 }
             }

输出：

5. TPT是实体框架支持三个继承映射中的一个，另外两个分别是Table per Hierarchy(TPH），和Table per Concrete Type(TPC），将在本章后面部分讨论。

　　TPT继承映射提供了数据库方面的灵活性，作为开发人员，我们可以很容易在模型中为新加的表添加派生类型。但是，每个派生类型都会涉及一个额外的join连接，这会降低系统的性能。 在真实的应用中，我们已经看到当派生类型很多时，使用TPT继承映射所带来的性能问题。

　　Table per hierarchy(TPH)继承映射。它将整个继承类型存储在一张单独的表中，他解决了TPT中的join连接问题，并带来了好的性能。但牺牲了数据库的灵活性。

　　Table per concrete（TPC)继承映射，它被实体框架于运行时所支持，但不被设计器支持。