《Entity Framework 6 Recipes》中文翻譯系列 (43) ------ 第八章 POCO之使用POCO加載實體

時間 2019-12-09

標籤 entity framework recipes 中文翻譯系列第八 poco 使用加載實體简体版

原文原文鏈接

翻譯的初衷以及爲何選擇《Entity Framework 6 Recipes》來學習，請看本系列開篇html

8-2 使用POCO加載關聯實體

問題框架

　　你想使用POCO預先加載關聯實體。ide

解決方案學習

　　假設你有如圖8-3所示的模型。ui

圖8-3. 一個包含實體Venue、Event和Competitor的模型this

　　實體使用POCO類，咱們想預先加載關聯實體（導航屬性）。並使用上下文對象中的Include()方法來實現。代碼清單8-4演示了使用Include()方法來實現咱們的要求。spa

代碼清單8-4. 使用Include()方法顯式加載導航屬性翻譯

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; }
    }

代碼清單8-4的輸出以下：code

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htm

　　當爲咱們的模型使用實體框架生成的代碼時，咱們使用上下文對象中的Include()方法，查詢並加載關聯實體，這些關聯實體多是實體的列表，多是一個單獨的實體對象。實體框架中一共有三種不一樣的方法來加載或查詢關聯實體： Eager Loading（預先加載）, Lazy Loading（延遲加載）和Explicit Loading（顯式加載）。示例中咱們使用Include()方法演示預先加載關聯實體。默認狀態下，實體框架是開啓延遲加載的，可是在這裏，咱們把它禁用了。爲了使用POCO顯式加載導航屬性，須要使用DbContext中的Include()方法。

8-3 使用POCO延遲加載

問題

　　你想使用POCO延遲加載關聯實體。

解決方案

　　假設你有如圖8-4所示的模型。

圖8-4. 一個關於交通罰單、違規車輛和違規細節的模型

　　啓用延遲加載，你不須要作任何事。它是實體框架的默認行爲。代碼清單8-5對此進行了演示。

代碼清單8-5. 實體類生成，屬性設置爲Virtual，這是實體框架的默認行爲

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; }
    }

代碼清單8-5的輸出以下：

Ticket: 1, Total Cost: $175.00
    20 MPH over the speed limit
    Broken tail light
Ticket: 2, Total Cost: $35.00
    Parking in a no parking zone

原理

　　當生成一個實體數據模型時，延遲加載被默認設置。導航屬性默認也被標記爲virtual。使用延遲加載，你不須要顯式地作任何事。

　　在上面的控制檯程序中，咱們沒有編寫別的代碼來加載Violation對象，它是Ticket對象的關聯對象。當你在代碼中訪問關聯實體時，延遲加載就生效了。它不須要上下文對象在第一次加載主實體時就加載關聯實體，不須要像上一節中使用Include()方法顯式加載關聯實體。

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