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JOIN not working; too many SELECTs querying single object

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SAFX

Post subject: JOIN not working; too many SELECTs querying single object

Posted: Sat Mar 12, 2011 12:12 pm

Newbie

Joined: Tue Feb 01, 2011 2:06 am
Posts: 8

The problem I've got is that Hibernate executes 3 SELECT statements when I query for a single, unique entity. I suspect my query and/or annotation are not correct.

A User has a UserStatusType and Region; simple one-to-one mappings.

Code:

@Entity
@Table( name = "USER" )
public class User extends {

   @Id
   @Column(name= "user_id")
   private Long userId;
   
   @OneToOne()
   @JoinColumn(name="user_status_type_id")
   protected UserStatusType userStatusType;

   @OneToOne()
   @JoinColumn(name="region_id")
   protected Region region;

My query code:

Code:

   Session session = currentSession();
   session.beginTransaction();
   User user = (User)session.createQuery(
      "from User as user where user_id = ?")
      .setLong(0, 42L)
      .uniqueResult();

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SAFX

Post subject: Re: JOIN not working; too many SELECTs querying single object

Posted: Sat Mar 12, 2011 2:51 pm

Newbie

Joined: Tue Feb 01, 2011 2:06 am
Posts: 8

Ok, I was able to get down to a single SELECT by enhancing the query, but now I'm getting a ClassCastException (see below).

Code:

   Session session = currentSession();
   session.beginTransaction();
   User user = (User)session.createQuery(
      "from User u, UserStatusType ust, UserRegion ur, " +
      " where u.user_id = ? AND u.userStatusType.id = ust.id AND u.region.id = ur.id")
      .setLong(0, 42L)
      .uniqueResult();

Hibernate debug logging confirms that only a single SELECT statement was used, that's good, but I keep getting this cast exception and don't know what the heck is going on.

Code:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.scd.dao.entity.NewUser
   at com.scd.entity.user.NewUserTest.selectNewUser(NewUserTest.java:111)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.lang.reflect.Method.invoke(Method.java:597)

I tried using setResultTransformer() but that resulted in a NullPointerException

Code:

   User user = (User)session.createQuery(
      "from User u, UserStatusType ust, UserRegion ur, " +
      " where u.user_id = ? AND u.userStatusType.id = ust.id AND u.region.id = ur.id")
      .setLong(0, 42L)
      .setResultTransformer( Transformers.aliasToBean(User.class))
      .uniqueResult();

Code:

java.lang.NullPointerException
   at org.hibernate.transform.AliasToBeanResultTransformer.initialize(AliasToBeanResultTransformer.java:112)
   at org.hibernate.transform.AliasToBeanResultTransformer.transformTuple(AliasToBeanResultTransformer.java:81)
   at org.hibernate.hql.HolderInstantiator.instantiate(HolderInstantiator.java:96)
   at org.hibernate.loader.hql.QueryLoader.getResultList(QueryLoader.java:423)

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SAFX

Post subject: Re: JOIN not working; too many SELECTs querying single object

Posted: Sat Mar 12, 2011 8:35 pm

Newbie

Joined: Tue Feb 01, 2011 2:06 am
Posts: 8

Ok, I figured this out, looks like I'm answering my own question, I hope it helps those reading this in the future.

I eliminated the ClassCastException by adding "Select u" to the query; this informs Hibernate to return that entity type; otherwise, an Object[] array is returned and that's what caused the cast error. I also used explicit "fetching" in the query to reduce the database operation to a single SELECT.

Code:

 User user = (User)session.createQuery(
      "Select u from User u " +
      " join FETCH u.userStatusType " +
      " join FETCH u.region " + 
      " where u.userId = ?")
      .setLong(0, 42L)
      .uniqueResult();

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