具有属性的关联实体的DAO类-如何实现？

Question

I have class CreatureType and Weapon (the best weapon to use against some creature) and I need to model the fact that a weapon has various efficiency against particular CreatureType. So I created the association entity CreatureTypeWeapon which has the efficiency attribute.

I coded it in this way (the principle is the same) http://uaihebert.com/?p=1674&page=22

The reason why I am asking this question is that I don't know how to implement dao class for the association entity.

I have the DAO class for CreatureType and I have the DAO class for Weapon which implements the interfaces with CRUD operations.

But I am somehow missing a way ho to deal with CRUD operations on the association entity. Let's say I have this DAO

public class CreatureTypeWeaponDAOImpl implements CreatureTypeWeaponDAO {

    @Override
    public void create(CreatureTypeWeapon creatureTypeWeapon) {
        // implementation
    }
    // ... other CRUD methods
}

I use it like this:

// this is like a person
CreatureType creatureType = new CreatureType();
creatureType.setName("alien");
creatureType.setMaxHitPoints(200);

// this is like a dog
Weapon weapon = new Weapon();           
weapon.setName("ak-47");

// this is like PersonDog
CreatureTypeWeapon creatureTypeWeapon = new CreatureTypeWeapon();
creatureTypeWeapon.setWeapon(weapon);
creatureTypeWeapon.setCreatureType(creatureType);
// this is like "adoptionDate" in the link posted
// 0.5 means when I hit it with ak-47 it takes half of its max hit points
// so I have to fire 2 times on it and it will die.
creatureTypeWeapon.setEfficiency(0.5);

// this is actually injected
CreatureTypeWeaponDAO dao = new CreatureTypeWeaponDAOImpl();
dao.create(creatureTypeWeapon);

But the question is, how to implement it? I am able to persist it normally, but let's say I am going to remove this relationship:

dao.remove(creatureTypeWeapon);

Which could be implemented like this

public class CreatureTypeWeaponDAOImpl implements CreatureTypeWeaponDAO {
    void remove(CreatureTypeWeapon arg) {
        CreatureTypeWeapon toRemove = em.find(/* WHAT TO PUT HERE */);
        em.remove(toRemove);
    }
}

Normally, I put there

em.find(CreatureTypeWeapon.class, arg.getId());

but there is not "id" in the association entity ....

UPDATE

Ok so this is the way how I am getting a CreatureTypeWeapon instances in the CreatureTypeWeaponDAOImpl

TypedQuery<CreatureTypeWeapon> query = 
    em.createQuery(
        "SELECT ctw FROM CreatureTypeWeapon ctw WHERE "
            + "creatureType = :creatureType "
            + "AND "
            + "weapon = :weapon", 
        CreatureTypeWeapon.class);

    CreatureType creatureTypePersisted = 
        em.getReference(CreatureType.class, creatureType.getId());
    Weapon weaponPersisted = em.getReference(Weapon.class, weapon.getId());

    query.setParameter("creatureType", creatureTypePersisted);
    query.setParameter("weapon", weaponPersisted);
    return query.getSingleResult();

Getting seems to be ok but when I am going to remove it in the remove method as simple as em.remove(creatureTypeWeapon); (as advised) I am getting this error:

javax.persistence.NoResultException: No entity found for query at org.hibernate.ejb.QueryImpl.getSingleResult(QueryImpl.java:291)
at cz.muni.fi.pa165.creatures.dao.impl.CreatureTypeWeaponDAOImpl.get(CreatureTypeWeaponDAOImpl.java:101)

This is a way how I am testing it:

public void testRemoveCreatureTypeWeapon() {

    Weapon w = new Weapon("ak-47");
    weaponDAO.create(w);

    CreatureType ct = new CreatureType("alien", 200);
    creatureTypeDAO.create(ct);

    assertNotNull(weaponDAO.get(w.getId()));
    assertNotNull(creatureTypeDAO.get(ct.getId()));

    CreatureTypeWeapon ctw = new CreatureTypeWeapon();
    ctw.setCreatureType(ct);
    ctw.setWeapon(w);
    ctw.setEffectivity(new Float(0.5));
    creatureTypeWeaponDAO.create(ctw);

    CreatureTypeWeapon ctw2 =
            creatureTypeWeaponDAO.get(ctw.getCreatureType(), ctw.getWeapon());

    ctw2.setCreatureType(ctw.getCreatureType());
    ctw2.setWeapon(ctw.getWeapon());

    creatureTypeWeaponDAO.remove(ctw);

    assertNull(creatureTypeWeaponDAO.get(ctw2.getCreatureType(), ctw2.getWeapon()));
} 

UPDATE #2

Ok I got it, the reason there was such error was that when query.getSingleResult() throws NoResultException, it means that when I try to get the entity after its removing in the test, there is no such record, so that method throws an execption, which is good. I handled it like this:

try {
    return query.getSingleResult();
} catch(NoResultException ex) {
    return null;
}

After doing so, it is possible to remove it normally, (it was doable even before it but I was confused and thought the error was there).

Answer 1

Couldn't you just delete the CreatureTypeWeapon entity? I think you don't need to do a find , just deleting the CreatureTypeWeapon entity it would remove the association with the CreatureType and Weapon .

public class CreatureTypeWeaponDAOImpl implements CreatureTypeWeaponDAO {
    void remove(CreatureTypeWeapon arg) {
        /*Some init codes here*/
        session.delete(arg);
    }
}

Answer 2

you need to define a new table that is the join of the table CreatureType and Weapon . For instance:

@Entity
public class CreatureTypeWeapon implements Serializable {

    @Id
    @ManyToOne
    @PrimaryKeyJoinColumn(name = "creature_id", referencedColumnName = "id")
    private CreatureType creatureType;

    @Id
    @ManyToOne
    @PrimaryKeyJoinColumn(name = "weapon_id", referencedColumnName = "id")
    private Weapon weapon;
}

you can then define your queries as follows:

Find by creature

"SELECT CreatureTypeWeapon WHERE creatureType = ?"

Find by weapon

"SELECT CreatureTypeWeapon WHERE weapon = ?"

Find by creature and weapon

"SELECT  CreatureTypeWeapon WHERE creatureType = ? AND weapon = ?"

Delete

em.delete(creatureTypeWeapon);