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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