《Entity Framework 6 Recipes》中文翻译系列 (43) ------ 第八章 POCO之使用POCO加载实体

翻译的初衷以及为什么选择《Entity Framework 6 Recipes》来学习，请看本系列开篇

8-2  使用POCO加载关联实体

问题

　　你想使用POCO预先加载关联实体。

解决方案

　　假设你有如图8-3所示的模型。

图8-3. 一个包含实体Venue、Event和Competitor的模型

　　实体使用POCO类，我们想预先加载关联实体（导航属性）。并使用上下文对象中的Include()方法来实现。代码清单8-4演示了使用Include()方法来实现我们的要求。

代码清单8-4. 使用Include()方法显式加载导航属性

class Program
    {
        static void Main(string[] args)
        {
            RunExample();
        }

        static void RunExample()
        {
            using (var context = new EFRecipesEntities())
            {
                var venue = new Venue { Name = "Sports and Recreational Grounds" };
                var event1 = new Event { Name = "Inter-school Soccer" };
                event1.Competitors.Add(new Competitor { Name = "St. Mary's School" });
                event1.Competitors.Add(new Competitor { Name = "City School" });
                venue.Events.Add(event1);
                context.Venues.Add(venue);
                context.SaveChanges();
            }
            using (var context = new EFRecipesEntities())
            {
                foreach (var venue in context.Venues.Include("Events").Include("Events.Competitors"))
                {
                    Console.WriteLine("Venue: {0}", venue.Name);
                    foreach (var evt in venue.Events)
                    {
                        Console.WriteLine("\tEvent: {0}", evt.Name);
                        Console.WriteLine("\t--- Competitors ---");
                        foreach (var competitor in evt.Competitors)
                        {
                            Console.WriteLine("\t{0}", competitor.Name);
                        }
                    }
                }
            }
            using (var context = new EFRecipesEntities())
            {
                foreach (var venue in context.Venues)
                {
                    Console.WriteLine("Venue: {0}", venue.Name);
                    context.Entry(venue).Collection(v => v.Events).Load();
                    foreach (var evt in venue.Events)
                    {
                        Console.WriteLine("\tEvent: {0}", evt.Name);
                        Console.WriteLine("\t--- Competitors ---");
                        context.Entry(evt).Collection(e => e.Competitors).Load();
                        foreach (var competitor in evt.Competitors)
                        {
                            Console.WriteLine("\t{0}", competitor.Name);
                        }
                    }
                }
            }

            Console.WriteLine("Enter input:");
            string line = Console.ReadLine();
            if (line == "exit")
            {
                return;
            };
        }
    }
    public partial class Competitor
    {
        public int CompetitorId { get; set; }
        public string Name { get; set; }
        public int EventId { get; set; }

        public virtual Event Event { get; set; }
    }
    public partial class Event
    {
        public Event()
        {
            this.Competitors = new HashSet<Competitor>();
        }

        public int EventId { get; set; }
        public string Name { get; set; }
        public int VenueId { get; set; }

        public virtual ICollection<Competitor> Competitors { get; set; }
        public virtual Venue Venue { get; set; }
    }
    public partial class Venue
    {
        public Venue()
        {
            this.Events = new HashSet<Event>();
        }

        public int VenueId { get; set; }
        public string Name { get; set; }

        public virtual ICollection<Event> Events { get; set; }
    }
    public partial class EFRecipesEntities : DbContext
    {
        public EFRecipesEntities()
            : base("name=EFRecipesEntities")
        {
　　　　　　　　this.Configuration.LazyLoadingEnabled = false;
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            throw new UnintentionalCodeFirstException();
        }

        public DbSet<Competitor> Competitors { get; set; }
        public DbSet<Event> Events { get; set; }
        public DbSet<Venue> Venues { get; set; }
    }

代码清单8-4的输出如下：

Venue: City Center Hall
    Event: All Star Boxing
    --- Competitors ---Big Joe Green
    Terminator Tim
Venue: Sports and Recreational Grounds
    Event: Inter-school Soccer
    --- Competitors ---St. Mary's School
    City School

原理

　　当为我们的模型使用实体框架生成的代码时，我们使用上下文对象中的Include()方法，查询并加载关联实体，这些关联实体可能是实体的列表，可能是一个单独的实体对象 。实体框架中一共有三种不同的方法来加载或查询关联实体： Eager Loading（预先加载）, Lazy Loading（延迟加载）和Explicit Loading（显式加载）。示例中我们使用Include()方法演示预先加载关联实体。默认状态下，实体框架是开启延迟加载的，但是在这里，我们把它禁用了。为了使用POCO显式加载导航属性，需要使用DbContext中的Include()方法。

8-3  使用POCO延迟加载

问题

　　你想使用POCO延迟加载关联实体。

解决方案

　　假设你有如图8-4所示的模型。

图8-4. 一个关于交通罚单、违规车辆和违规细节的模型

　　启用延迟加载 ，你不需要做任何事。它是实体框架的默认行为 。代码清单8-5对此进行了演示。

代码清单8-5. 实体类生成，属性设置为Virtual，这是实体框架的默认行为

class Program
    {
        static void Main(string[] args)
        {
            RunExample();
        }
        static void RunExample()
        {
            using (var context = new EFRecipesEntities())
            {
                var vh1 = new Vehicle { LicenseNo = "BR-549" };
                var t1 = new Ticket { IssueDate = DateTime.Parse("06/10/13") };
                var v1 = new Violation
                {
                    Description = "20 MPH over the speed limit",
                    Amount = 125M
                };
                var v2 = new Violation
                {
                    Description = "Broken tail light",
                    Amount = 50M
                };
                t1.Violations.Add(v1);
                t1.Violations.Add(v2);
                t1.Vehicle = vh1;
                context.Tickets.Add(t1);
                var vh2 = new Vehicle { LicenseNo = "XJY-902" };
                var t2 = new Ticket { IssueDate = DateTime.Parse("06/12/13") };
                var v3 = new Violation
                {
                    Description = "Parking in a no parking zone",
                    Amount = 35M
                };
                t2.Violations.Add(v3);
                t2.Vehicle = vh2;
                context.Tickets.Add(t2);
                context.SaveChanges();
            }
            using (var context = new EFRecipesEntities())
            {
                foreach (var ticket in context.Tickets)
                {
                    Console.WriteLine(" Ticket: {0}, Total Cost: {1}",
                      ticket.TicketId.ToString(),
                      ticket.Violations.Sum(v => v.Amount).ToString("C"));
                    foreach (var violation in ticket.Violations)
                    {
                        Console.WriteLine("\t{0}", violation.Description);
                    }
                }
            }
            Console.WriteLine("Enter input:");
            string line = Console.ReadLine();
            if (line == "exit")
            {
                return;
            };
        }
    }
    public partial class Ticket
    {
        public Ticket()
        {
            this.Violations = new HashSet<Violation>();
        }

        public int TicketId { get; set; }
        public int VehicleId { get; set; }
        public System.DateTime IssueDate { get; set; }

        public virtual Vehicle Vehicle { get; set; }
        public virtual ICollection<Violation> Violations { get; set; }
    }
    public partial class Vehicle
    {
        public Vehicle()
        {
            this.Tickets = new HashSet<Ticket>();
        }

        public int VehicleId { get; set; }
        public string LicenseNo { get; set; }

        public virtual ICollection<Ticket> Tickets { get; set; }
    }
    public partial class Violation
    {
        public int ViolationId { get; set; }
        public string Description { get; set; }
        public decimal Amount { get; set; }
        public int TicketId { get; set; }

        public virtual Ticket Ticket { get; set; }
    }
 public partial class EFRecipesEntities : DbContext
    {
        public EFRecipesEntities()
            : base("name=EFRecipesEntities")
        {
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            throw new UnintentionalCodeFirstException();
        }

        public DbSet<Ticket> Tickets { get; set; }
        public DbSet<Vehicle> Vehicles { get; set; }
        public DbSet<Violation> Violations { get; set; }
    }

代码清单8-5的输出如下：

Ticket: , Total Cost: $175.00
     MPH over the speed limit
    Broken tail light
Ticket: , Total Cost: $35.00
    Parking in a no parking zone

原理

　　当生成一个实体数据模型时，延迟加载被默认设置。导航属性默认也被标记为virtual。使用延迟加载，你不需要显式地做任何事。

　　在上面的控制台程序中，我们没有编写别的代码来加载Violation对象，它是Ticket对象的关联对象。当你在代码中访问关联实体时，延迟加载就生效了。它不需要上下文对象在第一次加载主实体时就加载关联实体，不需要像上一节中使用Include()方法显式加载关联实体。

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