| # 4. Structured objects It's time to extend our business domain with another class and see how db4o handles object interrelations. Let's give our pilot a vehicle.
`namespace com.db4o.f1.chapter2 { public class Car { string _model; Pilot _pilot; public Car(string model) { _model = model; _pilot = null; } public Pilot Pilot { get { return _pilot; } set { _pilot = value; } } public string Model { get { return _model; } } override public string ToString() { return _model + "[" + _pilot + "]"; } } }`
## 4.1. Storing structured objects To store a car with its pilot, we just call set() on our top level object, the car. The pilot will be stored implicitly.
`[storeFirstCar] Car car1 = new Car("Ferrari"); Pilot pilot1 = new Pilot("Michael Schumacher", 100); car1.Pilot = pilot1; db.set(car1);`
Of course, we need some competition here. This time we explicitly store the pilot before entering the car - this makes no difference.
`[storeSecondCar] Pilot pilot2 = new Pilot("Rubens Barrichello", 99); db.set(pilot2); Car car2 = new Car("BMW"); car2.Pilot = pilot2; db.set(car2);`
## 4.2. Retrieving structured objects ## 4.2.1. QBE To retrieve all cars, we simply provide a 'blank' prototype.
`[retrieveAllCarsQBE] Car proto = new Car(null); ObjectSet result = db.get(proto); listResult(result);`
OUTPUT: 2 BMW[Rubens Barrichello/99] Ferrari[Michael Schumacher/100]

We can also query for all pilots, of course.

`[retrieveAllPilotsQBE] Pilot proto = new Pilot(null, 0); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 2 Michael Schumacher/100 Rubens Barrichello/99

Now let's initialize our prototype to specify all cars driven by Rubens Barrichello.

`[retrieveCarByPilotQBE] Pilot pilotproto = new Pilot("Rubens Barrichello",0); Car carproto = new Car(null); carproto.Pilot = pilotproto; ObjectSet result = db.get(carproto); listResult(result);`

OUTPUT: 1 BMW[Rubens Barrichello/99]

What about retrieving a pilot by car? We simply don't need that - if we already know the car, we can simply access the pilot field directly.

## 4.2.2. Native Queries

Using native queries with constraints on deep structured objects is straightforward, you can do it just like you would in plain other code.
Let's constrain our query to only those cars driven by a Pilot with a specific name:

`[retrieveCarsByPilotNameNative] string pilotName = "Rubens Barrichello"; ObjectSet results = db.query(new retrieveCarsByPilotNamePredicate( pilotName)); listResult(results);`

OUTPUT: 1 BMW[Rubens Barrichello/99]

## 4.2.3. SODA Query API

In order to use SODA for querying for a car given its pilot's name we have to descend two levels into our query.

`[retrieveCarByPilotNameQuery] Query query = db.query(); query.constrain(typeof(Car)); query.descend("_pilot").descend("_name") .constrain("Rubens Barrichello"); ObjectSet result = query.execute(); listResult(result);`

OUTPUT: 1 BMW[Rubens Barrichello/99]

We can also constrain the pilot field with a prototype to achieve the same result.

`[retrieveCarByPilotProtoQuery] Query query = db.query(); query.constrain(typeof(Car)); Pilot proto = new Pilot("Rubens Barrichello", 0); query.descend("_pilot").constrain(proto); ObjectSet result = query.execute(); listResult(result);`

OUTPUT: 1 BMW[Rubens Barrichello/99]

We have seen that descending into a query provides us with another query. Starting out from a query root we can descend in multiple directions. In practice this is the same as ascending from one child to a parent and descending to another child. We can conclude that queries turn one-directional references in our objects into true relations. Here is an example that queries for "a Pilot that is being referenced by a Car, where the Car model is 'Ferrari'":

`[retrievePilotByCarModelQuery] Query carquery=db.query(); carquery.constrain(typeof(Car)); carquery.descend("_model").constrain("Ferrari"); Query pilotquery=carquery.descend("_pilot"); ObjectSet result=pilotquery.execute(); listResult(result);`

OUTPUT: 1 Michael Schumacher/100

## 4.3. Updating structured objects

To update structured objects in db4o, we simply call set() on them again.

`[updateCar] ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot = new Pilot("Somebody else", 0); db.set(found); result = db.get(new Car("Ferrari")); listResult(result);`

OUTPUT: 1 Ferrari[Somebody else/0]

Let's modify the pilot, too.

`[updatePilotSingleSession] ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot.AddPoints(1); db.set(found); result = db.get(new Car("Ferrari")); listResult(result);`

OUTPUT: 1 Ferrari[Somebody else/1]

Nice and easy, isn't it? But wait, there's something evil lurking right behind the corner. Let's see what happens if we split this task in two separate db4o sessions: In the first we modify our pilot and update his car:

`[updatePilotSeparateSessionsPart1] ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot.AddPoints(1); db.set(found);`

And in the second, we'll double-check our modification:

`[updatePilotSeparateSessionsPart2] ObjectSet result = db.get(new Car("Ferrari")); listResult(result);`

OUTPUT: 1 Ferrari[Somebody else/2]

Looks like we're in trouble: Why did the Pilot's points not change? What's happening here and what can we do to fix it?

## 4.3.1. Update depth

Imagine a complex object with many members that have many members themselves. When updating this object, db4o would have to update all its children, grandchildren, etc. This poses a severe performance penalty and will not be necessary in most cases - sometimes, however, it will.
So, in our previous update example, we were modifying the Pilot child of a Car object. When we saved the change, we told db4o to save our Car object and asumed that the modified Pilot would be updated. But we were modifying and saving in the same manner as we were in the first update sample, so why did it work before? The first time we made the modification, db4o never actually had to retreive the modified Pilot it returned the same one that was still in memory that we modified, but it never actually updated the database. The fact that we saw the modified value was, in fact, a bug. Restarting the application would show that the value was unchanged.
To be able to handle this dilemma as flexible as possible, db4o introduces the concept of update depth to control how deep an object's member tree will be traversed on update. The default update depth for all objects is 1, meaning that only primitive and String members will be updated, but changes in object members will not be reflected.
db4o provides means to control update depth with very fine granularity. For our current problem we'll advise db4o to update the full graph for Car objects by setting cascadeOnUpdate() for this class accordingly.

`[updatePilotSeparateSessionsImprovedPart1] Db4o.configure().objectClass(typeof(Car)) .cascadeOnUpdate(true);`

`[updatePilotSeparateSessionsImprovedPart2] ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot.AddPoints(1); db.set(found);`

`[updatePilotSeparateSessionsImprovedPart3] ObjectSet result = db.get(new Car("Ferrari")); listResult(result);`

OUTPUT: 1 Ferrari[Somebody else/3]

This looks much better.
Note that container configuration must be set before the container is opened.
We'll cover update depth as well as other issues with complex object graphs and the respective db4o configuration options in more detail in a later chapter.

## 4.4. Deleting structured objects

As we have already seen, we call delete() on objects to get rid of them.

`[deleteFlat] ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); db.delete(found); result = db.get(new Car(null)); listResult(result);`

OUTPUT: 1 BMW[Rubens Barrichello/99]

Fine, the car is gone. What about the pilots?

`[retrieveAllPilotsQBE] Pilot proto = new Pilot(null, 0); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 1 Rubens Barrichello/99

Ok, this is no real surprise - we don't expect a pilot to vanish when his car is disposed of in real life, too. But what if we want an object's children to be thrown away on deletion, too?

## 4.4.1. Recursive deletion

You may already suspect that the problem of recursive deletion (and perhaps its solution, too) is quite similar to our little update problem, and you're right. Let's configure db4o to delete a car's pilot, too, when the car is deleted.

`[deleteDeepPart1] Db4o.configure().objectClass(typeof(Car)) .cascadeOnDelete(true);`

`[deleteDeepPart2] ObjectSet result = db.get(new Car("BMW")); Car found = (Car)result.next(); db.delete(found); result = db.get(new Car(null)); listResult(result);`

OUTPUT: 0

Again: Note that all configuration must take place before the ObjectContainer is opened.
Let's have a look at our pilots again.

`[retrieveAllPilots] Pilot proto = new Pilot(null, 0); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 0

## 4.4.2. Recursive deletion revisited

But wait - what happens if the children of a removed object are still referenced by other objects?

`[deleteDeepRevisited] ObjectSet result = db.get(new Pilot("Michael Schumacher", 0)); Pilot pilot = (Pilot)result.next(); Car car1 = new Car("Ferrari"); Car car2 = new Car("BMW"); car1.Pilot = pilot; car2.Pilot = pilot; db.set(car1); db.set(car2); db.delete(car2); result = db.get(new Car(null)); listResult(result);`

OUTPUT: 1 Ferrari[null]

`[retrieveAllPilots] Pilot proto = new Pilot(null, 0); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 0

Houston, we have a problem - and there's no simple solution at hand. Currently db4o does not check whether objects to be deleted are referenced anywhere else, so please be very careful when using this feature.
Let's clear our database for the next chapter.

`[deleteAll] ObjectSet result = db.get(typeof(Object)); while(result.hasNext()){ db.delete(result.next()); }`

## 4.5. Conclusion

So much for object associations: We can hook into a root object and climb down its reference graph to specify queries. But what about multi-valued objects like arrays and collections? We will cover this in the next chapter .

## 4.6. Full source

`namespace com.db4o.f1.chapter2 { using System; using System.IO; using com.db4o; using com.db4o.f1; using com.db4o.query; public class StructuredExample : Util { public static void Main(String[] args) { File.Delete(Util.YapFileName); ObjectContainer db = Db4o.openFile(Util.YapFileName); try { storeFirstCar(db); storeSecondCar(db); retrieveAllCarsQBE(db); retrieveAllPilotsQBE(db); retrieveCarByPilotQBE(db); retrieveCarByPilotNameQuery(db); retrieveCarByPilotProtoQuery(db); retrievePilotByCarModelQuery(db); updateCar(db); updatePilotSingleSession(db); updatePilotSeparateSessionsPart1(db); db.close(); db=Db4o.openFile(Util.YapFileName); updatePilotSeparateSessionsPart2(db); db.close(); updatePilotSeparateSessionsImprovedPart1(db); db=Db4o.openFile(Util.YapFileName); updatePilotSeparateSessionsImprovedPart2(db); db.close(); db=Db4o.openFile(Util.YapFileName); updatePilotSeparateSessionsImprovedPart3(db); deleteFlat(db); db.close(); deleteDeepPart1(db); db=Db4o.openFile(Util.YapFileName); deleteDeepPart2(db); deleteDeepRevisited(db); } finally { db.close(); } } public static void storeFirstCar(ObjectContainer db) { Car car1 = new Car("Ferrari"); Pilot pilot1 = new Pilot("Michael Schumacher", 100); car1.Pilot = pilot1; db.set(car1); } public static void storeSecondCar(ObjectContainer db) { Pilot pilot2 = new Pilot("Rubens Barrichello", 99); db.set(pilot2); Car car2 = new Car("BMW"); car2.Pilot = pilot2; db.set(car2); } public static void retrieveAllCarsQBE(ObjectContainer db) { Car proto = new Car(null); ObjectSet result = db.get(proto); listResult(result); } public static void retrieveAllPilotsQBE(ObjectContainer db) { Pilot proto = new Pilot(null, 0); ObjectSet result = db.get(proto); listResult(result); } public static void retrieveCarByPilotQBE(ObjectContainer db) { Pilot pilotproto = new Pilot("Rubens Barrichello",0); Car carproto = new Car(null); carproto.Pilot = pilotproto; ObjectSet result = db.get(carproto); listResult(result); } public static void retrieveCarByPilotNameQuery(ObjectContainer db) { Query query = db.query(); query.constrain(typeof(Car)); query.descend("_pilot").descend("_name") .constrain("Rubens Barrichello"); ObjectSet result = query.execute(); listResult(result); } public static void retrieveCarByPilotProtoQuery(ObjectContainer db) { Query query = db.query(); query.constrain(typeof(Car)); Pilot proto = new Pilot("Rubens Barrichello", 0); query.descend("_pilot").constrain(proto); ObjectSet result = query.execute(); listResult(result); } public static void retrievePilotByCarModelQuery(ObjectContainer db) { Query carquery=db.query(); carquery.constrain(typeof(Car)); carquery.descend("_model").constrain("Ferrari"); Query pilotquery=carquery.descend("_pilot"); ObjectSet result=pilotquery.execute(); listResult(result); } public static void retrieveAllPilots(ObjectContainer db) { ObjectSet results = db.get(typeof(Pilot)); listResult(results); } public static void retrieveAllCars(ObjectContainer db) { ObjectSet results = db.get(typeof(Car)); listResult(results); } public class retrieveCarsByPilotNamePredicate : Predicate{ readonly string pilotName; public retrieveCarsByPilotNamePredicate(string pilotName){ this.pilotName = pilotName; } public bool Match(Car candidate){ return candidate.Pilot.Name == pilotName; } } public static void retrieveCarsByPilotNameNative(ObjectContainer db) { string pilotName = "Rubens Barrichello"; ObjectSet results = db.query(new retrieveCarsByPilotNamePredicate( pilotName)); listResult(results); } public static void updateCar(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot = new Pilot("Somebody else", 0); db.set(found); result = db.get(new Car("Ferrari")); listResult(result); } public static void updatePilotSingleSession(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot.AddPoints(1); db.set(found); result = db.get(new Car("Ferrari")); listResult(result); } public static void updatePilotSeparateSessionsPart1(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot.AddPoints(1); db.set(found); } public static void updatePilotSeparateSessionsPart2(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); listResult(result); } public static void updatePilotSeparateSessionsImprovedPart1(ObjectContainer db) { Db4o.configure().objectClass(typeof(Car)) .cascadeOnUpdate(true); } public static void updatePilotSeparateSessionsImprovedPart2(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); found.Pilot.AddPoints(1); db.set(found); } public static void updatePilotSeparateSessionsImprovedPart3(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); listResult(result); } public static void deleteFlat(ObjectContainer db) { ObjectSet result = db.get(new Car("Ferrari")); Car found = (Car)result.next(); db.delete(found); result = db.get(new Car(null)); listResult(result); } public static void deleteDeepPart1(ObjectContainer db) { Db4o.configure().objectClass(typeof(Car)) .cascadeOnDelete(true); } public static void deleteDeepPart2(ObjectContainer db) { ObjectSet result = db.get(new Car("BMW")); Car found = (Car)result.next(); db.delete(found); result = db.get(new Car(null)); listResult(result); } public static void deleteDeepRevisited(ObjectContainer db) { ObjectSet result = db.get(new Pilot("Michael Schumacher", 0)); Pilot pilot = (Pilot)result.next(); Car car1 = new Car("Ferrari"); Car car2 = new Car("BMW"); car1.Pilot = pilot; car2.Pilot = pilot; db.set(car1); db.set(car2); db.delete(car2); result = db.get(new Car(null)); listResult(result); } } }`

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