Where to provide entity relations: entities or services

[toelen]

Hi,

I have an application with a lot of entities with relations to each other. E.g cars, wheels, paint, seats, bolts, ...
The (client) application directly directly talks to a database and uses a office wide memcache, so caching is a concern as well. The object model is too large to keep everything in memory, so a form of lazy loading is needed. This is a GUI application so startup performance is an issue as well.

My question is where to draw the line between entities and their relations, are there any best practices.

Possibility one: make the entities responsible for the relations. In reality this demands that the entities keep a reference to the service

Car
.getWheels(){return carservice.getWheels(car);}
.getPaint(){return carservice.getPaint(car);}

Wheel
.getCar(){return wheelservice.getCar(wheel);}

Paint
.getCars(){return wheelservice.getCar(paint);}

Possibility two: relations are kept in the service. Entities are value objects only, keeping only a reference to the ID's of their children. Every time a child lookup is needed, the service should be called.

Car
.getID();
.getModel();
.getWheelIDs();

Wheel
.getID();
.getType();
.getCarID();

Paint
.getID();
.getColor();
.getCarID();

CarService
.getCars();
.getCar(id);

WheelService
.getWheel(id);
.getWheels(Car);
.getWheels(Car);

PaintService
.getPaints();
.getPaint(Car);

Regards,
Leen

[byte23]

Hmm I'd do neither nor. I would have my Entities which contain data AND relationships. The definition of a service instead should be to implement a business function or process.

So my structure would be:

public class Wheel {
...
}

public class Car {
  private List<Wheel> wheels;
  public void addWheel(Wheel wheel) { wheels.add(wheel); }
}

public class AssemblyService {
  public void assembleWheels(...) {
     // some sophisticated business logic...
     Wheel w = new Wheel();
     w.setSize(17);
     w.setType("Snowtires");
     ...
     Car c = getCarFromDatabase(); 
     c.addWheel(w);
     ...
  }
}

You should not have a dependency from the entity to the service, only the other way round. Talking best practices there should also be a DAO inbetween, which provides functionality to map your entities to the database. You might also consider to use an object-relational framework which can support you in a variety of problems, like resolving object relations (like get a car from database, fetch all assigned wheels from database, create wheel objects, create a car object and assign the wheel objects to the car object) and so forth.

[einfopedia]

I think this method is best one.Thanx for sahring.