Agfdhyk

This is applicable to any language, but I'm using Java for this so...

public class Egg {

    ...

}



public class BirdNest {



    private List<Egg> eggs;



    ...

}

I also have database entities which match these classes. Basically, BirdNest has 1:M relationship with Egg.

To perform persistence/retrieval actions on these classes I also have a BirdNestService and an EggService.

Say I want to retrieve a list of eggs from a given bird nest.

I could have a method like List<Egg> getEggs (int birdNestId);

My question is, which service should methods like this belong to?

It's performing operations based on a particular bird nest so you could argue it should be part of the BirdNestService.

Then again you could argue the item it is retrieving are Egg's so it should belong to the EggService.

If you put it into EggService then this service knows about a BirdNest concept (birdNestId) which is not something you want.

So the answer is BirdNestService.

I think it is the part of BirdNestService because Egg is just an entity and it does't depend upon Nest. But Nest contains Eggs otherwise Nest is useless without the Eggs. Nest depend upon Eggs but eggs does't depend upon Nest. In my opinion Nest must have information for which eggs he have and it already contain the list of eggs as you described in the class and eggs doesn't need to remember their Nest Id unless you have a very complex scenario in which you want to find nest Id by their Eggs or some specific requirement.

BirdNestService shouldn't need a method like List<Egg> getEggs (int birdNestId);.

If you want to retrieve eggs from a given birdnest, then it should be simple like that: List<Egg> myEggs = myBirdNest.getEggs()
BirdNestService should have operations (methods) you want to use on birdnests, e.g. BirdNest buildBirdNest(int amountStems, Bird byBird).

I would say it should be the EggService.

The reason is currently your query only needs Eggs by Birds Nest but that is probably not going to be the case always. Tomorrow you may need to find eggs based on certain characteristics for example size, color etc...

Hence I would go for EggService because after all you are trying to identify Eggs with a certain characteristic - in this case belonging to a certain birdnest.

List getEggs (int birdNestId) method is asking for eggs contained in a particular nest. The way i think about it :

• Conceptually a bird nest stores handles(ids) for eggs. From the database perspective, foreign keys to egg table. Actual eggs are stored in egg table.


• Hence it makes more sense to ask BirdNestService for eggs of a nest which can do some validations like whether it's a valid nest id or not and then retrieve the handles of eggs.


• It can then consult EggService to give a list of eggs matching a list of ids.

Hence, i would prefer to have List getEggs (int birdNestId) method method in BirdNestService and will add a method in EggService like this :
List getEggs(List eggIds).

But again, there is no right or wrong answer. It will depend on what makes your design clean, coherent and maintainable.

You retrieve attribute of the entity, so it's part of the nest. No brainer.

Now, if you had attribute birdnestid and you filtered your egg database by that attribute it would be part of egg entity, hence you would put it into egg service.

Rule of thumb: it's okay as long as it's an attribute.

In that way you can have both methods in two different services.

This also applies to multilayered attributes: it depends on the entity you use as base

Which should you use? As a rule of thumb, use top to bottom approach: always go for parent when you need the child