Trouble with Cascading, Unidirectional @OneToMany

Daythryl · **Joined:** Mon Dec 11, 2006 8:28 am **Posts:** 13

I'm obviously not understanding something about how this works. Hopefully I can demonstrate my confusion consicely so that someone can elucidate:

I have two classes as follows:

Code:

@Entity
public class GenOne {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    long id;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "genone")
    Set<GenTwo> gentwos = new HashSet<GenTwo>();
}

@Entity
public class GenTwo {
    @Id
    @GeneratedValue (strategy = GenerationType.AUTO)
    private long id;

    private long genone;
}

I try to create a GenOne and put some GenTwos into it, thus:

Code:

            GenOne g1 = new GenOne();
            for (int i = 0; i < 3; i++) g1.gentwos.add(new GenTwo());
            session.save(g1);

On the save I get an exception, because when trying to insert a record for the first GenTwo, Hibernate sets its foreign key (genone) to zero, which fails the foreign-key constraint. I would have expected Hibernate to first insert the GenOne, get its id and then use that to fill the genone values in the GenTwo records. What am I doing wrong?

TIA

Michael

Masterplan79th · **Joined:** Fri Jun 26, 2009 6:59 am **Posts:** 23

try removing your @JoinColumn annotation and replacing:
@OneToMany(cascade = CascadeType.ALL)
with
@OneToMany(cascade = CascadeType.ALL, mappedBy="genone")

add @Column(name = "genone") to your attribute in GenTwo representing the foreign key.

this is how i do my unidirectional One-to-Many mappings.

Daythryl · **Joined:** Mon Dec 11, 2006 8:28 am **Posts:** 13

That sounds like it could work, and thank you for the response, but won't that make it bidirectional? There should be a way to do it as a unidirectional relationship.

archetyp3 · **Joined:** Mon Aug 24, 2009 4:59 pm **Posts:** 2

Did you try to add fetch type?

Daythryl · **Joined:** Mon Dec 11, 2006 8:28 am **Posts:** 13

I just tried it with both EAGER and LAZY, with no change in behavior.

Daythryl · **Joined:** Mon Dec 11, 2006 8:28 am **Posts:** 13

I also tried your suggestion about the mapping, and threw in a Hibernate Cascade annotation for the hell of it, so now my code looks like this:

Code:

@Entity
public class GenOne {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    long id;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "genone", fetch = FetchType.EAGER)
    @org.hibernate.annotations.Cascade(
            value = org.hibernate.annotations.CascadeType.ALL
    )
    Set<GenTwo> gentwos = new HashSet<GenTwo>();
}

@Entity
public class GenTwo {
    @Id
    @GeneratedValue (strategy = GenerationType.AUTO)
    private long id;

    @Column(name = "genone") 
    private long genone;
}

Still no joy. I'm now wondering whether it would work if I were to use a different key-generation strategy for GenOne, so that Hibernate won't have to get the autogenerated id back from the GenOne insertion to place into the foreign key field in GenTwo.

Daythryl · **Joined:** Mon Dec 11, 2006 8:28 am **Posts:** 13

Okay, I got it now (with some help from my friend Jeremy Barkan):

Code:

@Entity
public class GenOne {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    long id;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @org.hibernate.annotations.Cascade(value = org.hibernate.annotations.CascadeType.ALL)
    @JoinColumn(name="genone")
    Set<GenTwo> gentwos = new HashSet<GenTwo>();
}

@Entity
public class GenTwo {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
}

Hibernate adds the genone foreign-key field to the GenTwo table w/o my having to define it in GenTwo -- in fact, if I define it there that throws everything off.

So now I can get back to work, but I'd like to point two things out:

updates the foreign key of each child record in an individual update statement

really

nullable = false

still updates it to the same value in individual updates

Code:

SQL  - insert into GenOne values ( )
SQL  - insert into GenTwo (genone) values (?)
LongType  - binding '7' to parameter: 1
SQL  - insert into GenTwo (genone) values (?)
LongType  - binding '7' to parameter: 1
SQL  - insert into GenTwo (genone) values (?)
LongType  - binding '7' to parameter: 1
SQL  - update GenTwo set genone=? where id=?
LongType  - binding '7' to parameter: 1
LongType  - binding '1' to parameter: 2
SQL  - update GenTwo set genone=? where id=?
LongType  - binding '7' to parameter: 1
LongType  - binding '2' to parameter: 2
SQL  - update GenTwo set genone=? where id=?
LongType  - binding '7' to parameter: 1
LongType  - binding '3' to parameter: 2