Question

我有类CreatureType和Weapon（对某些生物使用的最佳武器），我需要模拟武器对特定CreatureType具有各种效率的事实。所以我创建了具有效率属性的关联实体CreatureTypeWeapon。

我用这种方式编码（原理是相同的）http://uaihebert.com/?p=1674&page=22

我问这个问题的原因是我不知道如何为关联实体实现dao类。

我有用于CreatureType的DAO类，我有武器的DAO类，它实现了与CRUD操作的接口。

但我在某种程度上错过了处理关联实体上的CRUD操作的方法。假设我有这个DAO

public class CreatureTypeWeaponDAOImpl implements CreatureTypeWeaponDAO {

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

我这样用：

// 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);

但问题是，如何实施呢？我能够正常地坚持下去，但是我要说我要删除这种关系：

dao.remove(creatureTypeWeapon);

可以像这样实施

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

通常，我放在那里

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

但是关联实体中没有“id”....

更新

好的，这就是我在CreatureTypeWeaponDAOImpl中获取CreatureTypeWeapon实例的方式

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();

似乎没问题，但是当我要在remove方法中删除它时，就像em.remove（creatureTypeWeapon）一样简单; （按照建议）我收到了这个错误：

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)

这是我测试它的方式：

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()));
}

更新＃2

好吧我明白了，之所以出现这样的错误是因为当query.getSingleResult（）抛出NoResultException时，这意味着当我尝试在测试中删除实体后获取实体时，没有这样的记录，所以抛出一个execption，这很好。我像这样处理它：

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

这样做之后，可以正常删除它（它甚至可以在它之前实现，但我很困惑并且认为错误存在）。

Answer 1

你能不能删除CreatureTypeWeapon实体？我认为您不需要执行查找，只需删除CreatureTypeWeapon实体即可删除与CreatureType和Weapon的关联。

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

Answer 2

您需要定义一个新表，该表是表CreatureType和Weapon的连接。例如：

@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;
}

然后您可以按如下方式定义查询：

通过生物找到

"SELECT CreatureTypeWeapon WHERE creatureType = ?"

武器查找

"SELECT CreatureTypeWeapon WHERE weapon = ?"

通过生物和武器找到

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

删除

em.delete(creatureTypeWeapon);

具有属性的关联实体的DAO类 - 如何实现它？

2 个答案: