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