Entity Framework 6 Recipes 2nd Edition（11-5）译 -> 从”模型定义”函数返回一个匿名类型
11-5. 从”模型定义”函数返回一个匿名类型
问题
想创建一个返回一个匿名类型的”模型定义”函数
解决方案
假设已有游客(Visitor) 预订(reservation)房间(hotel ) 的模型,如Figure 11-5所示.
Figure 11-5. A model for hotel reservations
想要返回每位游客房间预订条数和带来的总收入.因为很多地方需要这些信息,所以想要创建一个”模型定义”函数,接受一个查询参数,返回一个包含游客合计信息的匿名类型的集合:
2. 把Listing 11-9 中的代码,插入到.edmx文件的概念模型里<Schema> 标签下,这样我们就定义好了函数.
Listing 11-9. The VisitorSummary() Model-Defined Function
<Function Name="VisitorSummary">
<Parameter Name="StartDate" Type="Edm.DateTime" />
<Parameter Name="Days" Type="Edm.Int32" />
<ReturnType>
<CollectionType>
<RowType>
<Property Name="Name" Type="Edm.String" />
<Property Name="TotalReservations" Type="Edm.Int32" />
<Property Name="BusinessEarned" Type="Edm.Decimal" />
</RowType>
</CollectionType>
</ReturnType>
<DefiningExpression>
Select
r.Visitor.Name,
COUNT(r.ReservationId) as TotalReservations,
SUM(r.Cost) as BusinessEarned
from EFRecipesEntities.Reservations as r
where r.ReservationDate between StartDate and
AddDays(StartDate,Days)
group by r.Visitor.Name
</DefiningExpression>
</Function>
3. 接下来插入和查询模型,代码如 Listing 11-10所示:.
Listing 11-10. Querying the Model Using the VistorySummary() Model-Defined Function
class Program
{
static void Main(string[] args)
{
RunExample();
}
static void RunExample()
{
using (var context = new EFRecipesEntities())
{
var hotel = new Hotel { Name = "Five Seasons Resort" };
var v1 = new Visitor { Name = "Alex Stevens" };
var v2 = new Visitor { Name = "Joan Hills" };
var r1 = new Reservation
{
Cost = 79.99M,
Hotel = hotel,
ReservationDate = DateTime.Parse("2/19/2010"),
Visitor = v1
};
var r2 = new Reservation
{
Cost = 99.99M,
Hotel = hotel,
ReservationDate = DateTime.Parse("2/17/2010"),
Visitor = v2
};
var r3 = new Reservation
{
Cost = 109.99M,
Hotel = hotel,
ReservationDate = DateTime.Parse("2/18/2010"),
Visitor = v1
};
var r4 = new Reservation
{
Cost = 89.99M,
Hotel = hotel,
ReservationDate = DateTime.Parse("2/17/2010"),
Visitor = v2
};
context.Hotels.Add(hotel);
context.SaveChanges();
}
using (var context = new EFRecipesEntities())
{
Console.WriteLine("Using eSQL...");
var esql = @"Select value v from
EFRecipesModel.VisitorSummary(DATETIME'2010-02-16 00:00', 7) as v";
var objectContext = (context as IObjectContextAdapter).ObjectContext;
var visitors = objectContext.CreateQuery<DbDataRecord>(esql);
foreach (var visitor in visitors)
{
Console.WriteLine("{0}, Total Reservations: {1}, Revenue: {2:C}",
visitor["Name"], visitor["TotalReservations"],
visitor["BusinessEarned"]);
}
}
using (var context = new EFRecipesEntities())
{
Console.WriteLine();
Console.WriteLine("Using LINQ...");
//译注:在我的EF6.1.1.3下,这句查询会出现异常
var visitors = from v in
context.VisitorSummary(DateTime.Parse("2/16/2010"), 7)
select v;
foreach (var visitor in visitors)
{
Console.WriteLine("{0}, Total Reservations: {1}, Revenue: {2:C}",
visitor["Name"], visitor["TotalReservations"],
visitor["BusinessEarned"]);
}
}
}
}
partial class EFRecipesEntities
{
[EdmFunction("EFRecipesModel", "VisitorSummary")]
public IQueryable<DbDataRecord> VisitorSummary(DateTime StartDate, int Days)
{
var objectContext = (this as IObjectContextAdapter).ObjectContext;
return objectContext.CreateQuery<DbDataRecord>(
Expression.Call(Expression.Constant(this),
(MethodInfo)MethodInfo.GetCurrentMethod(),
new Expression[] { Expression.Constant(StartDate),
Expression.Constant(Days) }
).ToString());
}
}
上面Listing 11-10 的代码输出如下:
Using eSQL...
Alex Stevens, Total Reservations: 2, Revenue: $189.98
Joan Hills, Total Reservations: 2, Revenue: $189.98
Using LINQ...
Alex Stevens, Total Reservations: 2, Revenue: $189.98
Joan Hills, Total Reservations: 2, Revenue: $189.98
它是如何工作的?
在Listing 11-9里, 在VisitorSummary() 函数定义里,我们按实体的导航属性visitor进行分组,我们用eSQL Count() 的 函数计算每位游客的预订房间数量,用Sum() 把每位游客付的租金合计.
在这个函数里,我们把结果组织成:: Name, TotalReservations, 和BusinessEarned. 此处我们使用了<CollectionType> 和<RowType> 标签来指明返回类型. 在运行时里,用包含DbDataRecords的集合
为了使该函数能在LINQ查询中使用,我们创建了运行时方法(返回IQueryable<DbDataRecord>.) .和前面的小节一样,我们用EdmFunction()特性修饰该方法.. 因为需要返回一个IQueryable<T>, 所以我们需要在这个方法体中包含函数调用,以便它能用在查询表达式树中.
此外,我们需要在ObjectContext中访问QueryProvider来返回一个IQueryable<T>,所以我们需要在EFRecipesEntities类中实现这个方法.
附：创建示例用到的数据库的脚本文件