实体：
public class Room
{
public Room()
{
this.Reservations = new HashSet<Reservation>();
}
public int Id { get; set; }
public decimal Rate { get; set; }
public int HotelId { get; set; }
public virtual Hotel Hotel { get; set; }
public virtual ICollection<Reservation> Reservations { get; set; }
}
public class Hotel
{
public Hotel()
{
this.Rooms = new HashSet<Room>();
}
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Room> Rooms { get; set; }
}
public class Reservation
{
public int Id { get; set; }
public DateTime StartDate { get; set; }
public DateTime EndDate { get; set; }
public string ContactName { get; set; }
public int RoomId { get; set; }
public virtual Room Room { get; set; }
}
public class ExecutiveSuite : Room
{
}
public class DataContext : DbContext
{
public DbSet<Hotel> Hotels { get; set; }
public DbSet<Reservation> Reservations { get; set; }
public DbSet<Room> Rooms { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Room>()
.HasKey(r => r.Id)
.HasRequired(r => r.Hotel)
.WithMany(r => r.Rooms)
.HasForeignKey(r => r.HotelId);
modelBuilder.Entity<Hotel>()
.HasKey(h => h.Id);
modelBuilder.Entity<Room>()
.HasMany(r => r.Reservations)
.WithRequired(r => r.Room)
.HasForeignKey(r => r.RoomId);
}
}
客户端代码（控制台应用程序）：
static void Main(string[] args)
{
// initialize and seed the database
using (var context = new DataContext())
{
var hotel = new Hotel { Name = "Grand Seasons Hotel" };
var r101 = new Room { Rate = 79.95M, Hotel = hotel };
var es201 = new ExecutiveSuite { Rate = 179.95M, Hotel = hotel };
var es301 = new ExecutiveSuite { Rate = 299.95M, Hotel = hotel };
var res1 = new Reservation
{
StartDate = DateTime.Parse("3/12/2010"),
EndDate = DateTime.Parse("3/14/2010"),
ContactName = "Roberta Jones",
Room = es301
};
var res2 = new Reservation
{
StartDate = DateTime.Parse("1/18/2010"),
EndDate = DateTime.Parse("1/28/2010"),
ContactName = "Bill Meyers",
Room = es301
};
var res3 = new Reservation
{
StartDate = DateTime.Parse("2/5/2010"),
EndDate = DateTime.Parse("2/6/2010"),
ContactName = "Robin Rosen",
Room = r101
};
es301.Reservations.Add(res1);
es301.Reservations.Add(res2);
r101.Reservations.Add(res3);
hotel.Rooms.Add(r101);
hotel.Rooms.Add(es201);
hotel.Rooms.Add(es301);
context.Hotels.Add(hotel);
context.SaveChanges();
}
using (var context = new DataContext())
{
context.Configuration.LazyLoadingEnabled = false;
// Assume we have an instance of hotel
var hotel = context.Hotels.First();
// Explicit loading with Load() provides opportunity to filter related data
// obtained from the Include() method
context.Entry(hotel)
.Collection(x => x.Rooms)
.Query()
.Include(y => y.Reservations)
.Where(y => y is ExecutiveSuite && y.Reservations.Any())
.Load();
Console.WriteLine("Executive Suites for {0} with reservations", hotel.Name);
foreach (var room in hotel.Rooms)
{
Console.WriteLine("\nExecutive Suite {0} is {1} per night", room.Id,
room.Rate.ToString("C"));
Console.WriteLine("Current reservations are:");
foreach (var res in room.Reservations.OrderBy(r => r.StartDate))
{
Console.WriteLine("\t{0} thru {1} ({2})", res.StartDate.ToShortDateString(),
res.EndDate.ToShortDateString(), res.ContactName);
}
}
}
Console.WriteLine("Press <enter> to continue...");
Console.ReadLine();
}
using ( var context = new DataContext() )
{
//context.Configuration.LazyLoadingEnabled = false;
// Assume we have an instance of hotel
var hotel = context.Hotels.First();
var rooms = context.Rooms.Include( r => r.Reservations ).Where( r => r is ExecutiveSuite && r.Reservations.Any() ).Where( r => r.Hotel.Id == hotel.Id );
Console.WriteLine( "Executive Suites for {0} with reservations", hotel.Name );
foreach ( var room in hotel.Rooms )
{
Console.WriteLine( "\nExecutive Suite {0} is {1} per night", room.Id,
room.Rate.ToString( "C" ) );
Console.WriteLine( "Current reservations are:" );
foreach ( var res in room.Reservations.OrderBy( r => r.StartDate ) )
{
Console.WriteLine( "\t{0} thru {1} ({2})", res.StartDate.ToShortDateString(),
res.EndDate.ToShortDateString(), res.ContactName );
}
}
}
我尝试进行投影并将其放入匿名对象中：
var hotel = context.Hotels.Select(h =>
new
{
Id = h.Id,
Name = h.Name,
Rooms = h.Rooms.Where(r => r.Reservations is ExecutiveSuite && r.Reservations.Any())
}).First();
但是我遇到了一个异常："DbIsOfExpression需要一个表达式参数，其多态结果类型与类型参数兼容。"
现在，如果你注意到了，我用两种不同的方式实现了它，第一种方式是通过显式加载相关实体，第二种方式是通过使用两个不同的查询，我的问题是，是否有一种方法可以仅使用一次从数据库中获取数据，并过滤我“包含”的实体对象图？