标签：poco lin 数据库 type context exp 代码 last --
1.让我们假设你有如图8-1中的表，Employee表包含hourly employees 和salaried employees的行。列EmployeeType作为鉴别列，鉴别这两种员工类型的行。 当EmployeType为1时，这一行代表一个专职员工(salaried or full-time employee)，当值为2时，这一行代码一个钟点工（hourly employee).
图8-1
2.右键你的项目，添加上图中的poco实体。如下：
[Table("Employee", Schema = "example8")] public abstract class Employee { [Key] [DatabaseGenerated(DatabaseGeneratedOption.Identity)] public int EmployeeId { get; protected set; } public string FirstName { get; set; } public string LastName { get; set; } }
public class FullTimeEmployee : Employee { public decimal? Salary { get; set; } }
public class HourlyEmployee : Employee { public decimal? Wage { get; set; } }
3.创建一个继承自DbContext的上下文对象EF6RecipesContext；
public class EF6RecipesContext : DbContext { public DbSet<Employee> Employees { get; set; } public EF6RecipesContext() : base("name=EF6CodeFirstRecipesContext") { } protected override void OnModelCreating(DbModelBuilder modelBuilder) { base.OnModelCreating(modelBuilder); modelBuilder.Entity<Employee>() .Map<FullTimeEmployee>(m => m.Requires("EmployeeType").HasValue(1)) .Map<HourlyEmployee>(m => m.Requires("EmployeeType").HasValue(2));//如果去掉这两个map，会发现程序也能正常运行，并且在Employee表中生成了一个额外属性“Discriminator” } }
4.测试代码：
1 using (var context = new EF6RecipesContext()) 2 { 3 var fte = new FullTimeEmployee 4 { 5 FirstName = "Jane", 6 LastName = "Doe", 7 Salary = 71500M 8 }; 9 context.Employees.Add(fte); 10 fte = new FullTimeEmployee 11 { 12 FirstName = "John", 13 LastName = "Smith", 14 Salary = 62500M 15 }; 16 context.Employees.Add(fte); 17 var hourly = new HourlyEmployee 18 { 19 FirstName = "Tom", 20 LastName = "Jones", 21 Wage = 8.75M 22 }; 23 context.Employees.Add(hourly); 24 context.SaveChanges(); 25 } 26 using (var context = new EF6RecipesContext()) 27 { 28 Console.WriteLine("--- All Employees ---"); 29 foreach (var emp in context.Employees) 30 { 31 bool fullTime = emp is HourlyEmployee ? false : true; 32 Console.WriteLine("{0} {1} ({2})", emp.FirstName, emp.LastName, 33 fullTime ? "Full Time" : "Hourly"); 34 } 35 Console.WriteLine("--- Full Time ---"); 36 foreach (var fte in context.Employees.OfType<FullTimeEmployee>()) 37 { 38 Console.WriteLine("{0} {1}", fte.FirstName, fte.LastName); 39 } 40 Console.WriteLine("--- Hourly ---"); 41 foreach (var hourly in context.Employees.OfType<HourlyEmployee>()) 42 { 43 Console.WriteLine("{0} {1}", hourly.FirstName, hourly.LastName); 44 } 45 }
输出如下：
--- All Employees ---Jane Doe (Full Time) John Smith (Full Time) Tom Jones (Hourly) --- Full Time ---Jane Doe John Smith --- Hourly ---Tom Jones
5.注意看步骤3中的注释，如果注释掉“OnModelCreating”中的内容，查询Employee生成的sql如下：
SELECT
[Extent1].[Discriminator] AS [Discriminator],
[Extent1].[EmployeeId] AS [EmployeeId],
[Extent1].[FirstName] AS [FirstName],
[Extent1].[LastName] AS [LastName],
[Extent1].[Salary] AS [Salary],
[Extent1].[Wage] AS [Wage]
FROM [example8].[Employee] AS [Extent1]
WHERE [Extent1].[Discriminator] IN (N‘FullTimeEmployee‘,N‘HourlyEmployee‘)
其实“TPT”与“TPH”的关系可以简单的理解成，是否子类与父类映射到同一张表。“TPT”相对灵活，单查询时使用了join，性能自然而然会有稍微影响；“TPH” 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。
entityframework学习笔记--008-实体数据建模基础之继承关系映射TPH
标签：poco lin 数据库 type context exp 代码 last --
原文地址：http://www.cnblogs.com/renjing/p/6138721.html
