For a simple example, let's take a Car class keeping a history of SensorReadout instances in a List member. Now imagine that we wanted to retrieve all cars that have assembled an even number of history entries. A quite contrived and seemingly trivial example, however, it gets us into trouble: Collections are transparent to the query API, it just 'looks through' them at their respective members.
So how can we get this done? Let's implement an Evaluation that expects the objects passed in to be instances of type Car and checks their history size.
01/* Copyright (C) 2004 - 2007 db4objects Inc. http://www.db4o.com */ 02
using Db4objects.Db4o.Query; 03
04
namespace Db4objects.Db4odoc.Evaluations 05
{ 06
public class EvenHistoryEvaluation : IEvaluation 07
{ 08
public void Evaluate(ICandidate candidate) 09
{ 10
Car car=(Car)candidate.GetObject(); 11
candidate.Include(car.History.Count % 2 == 0); 12
} 13
} 14
}
01' Copyright (C) 2004 - 2007 db4objects Inc. http://www.db4o.com 02
Imports Db4objects.Db4o.Query 03
04
Namespace Db4objects.Db4odoc.Evaluations 05
Public Class EvenHistoryEvaluation 06
Implements IEvaluation 07
Public Sub Evaluate(ByVal candidate As ICandidate) Implements IEvaluation.Evaluate 08
Dim car As Car = DirectCast(candidate.GetObject(), Car) 09
candidate.Include(car.History.Count Mod 2 = 0) 10
End Sub 11
12
End Class 13
End Namespace
To test it, let's add two cars with history sizes of one and two respectively:
01private static void StoreCars(IObjectContainer db) 02
{ 03
Pilot pilot1 = new Pilot("Michael Schumacher", 100); 04
Car car1 = new Car("Ferrari"); 05
car1.Pilot = pilot1; 06
car1.Snapshot(); 07
db.Set(car1); 08
Pilot pilot2 = new Pilot("Rubens Barrichello", 99); 09
Car car2 = new Car("BMW"); 10
car2.Pilot = pilot2; 11
car2.Snapshot(); 12
car2.Snapshot(); 13
db.Set(car2); 14
}
01Private Shared Sub StoreCars(ByVal db As IObjectContainer) 02
Dim pilot1 As Pilot = New Pilot("Michael Schumacher", 100) 03
Dim car1 As Car = New Car("Ferrari") 04
car1.Pilot = pilot1 05
car1.Snapshot() 06
db.Set(car1) 07
Dim pilot2 As Pilot = New Pilot("Rubens Barrichello", 99) 08
Dim car2 As Car = New Car("BMW") 09
car2.Pilot = pilot2 10
car2.Snapshot() 11
car2.Snapshot() 12
db.Set(car2) 13
End Sub
and run our evaluation against them:
1private static void QueryWithEvaluation(IObjectContainer db) 2
{ 3
IQuery query = db.Query(); 4
query.Constrain(typeof (Car)); 5
query.Constrain(new EvenHistoryEvaluation()); 6
IObjectSet result = query.Execute(); 7
ListResult(result); 8
}
1Private Shared Sub QueryWithEvaluation(ByVal db As IObjectContainer) 2
Dim query As IQuery = db.Query() 3
query.Constrain(GetType(Car)) 4
query.Constrain(New EvenHistoryEvaluation()) 5
Dim result As IObjectSet = query.Execute() 6
ListResult(result) 7
End Sub