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; }
    }

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

[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; }
    }

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)
        {

        }
    }

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 = "76106"
                };
                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("
--- All Businesses ---");
                foreach (var b in context.Businesses)
                {
                    Console.WriteLine("{0} (#{1}).{2}", b.Name, b.LicenseNumber,b.BusinessId);
                }

                Console.WriteLine("
--- 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("
--- 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。