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Help with Bidirectional OneToMany List using Annotations

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alexkwolfe

Post subject: Help with Bidirectional OneToMany List using Annotations

Posted: Fri Jun 24, 2005 2:37 pm

Newbie

Joined: Fri Jun 11, 2004 4:58 pm
Posts: 9

I'm having some trouble mapping a bidirectional one to many list using Hibernate 3.0.5, Annotations Beta 2, and hsqldb 1.7.2. Specifically, the index column is null for every new record. What have I done incorrectly?

Code:

@Entity
public class FieldTypeAttribute {
   ...
   @ManyToOne
   @JoinColumn(name="fieldTypeAttribute_id")
   public FieldType getType() {
        return type;
   }

   public void setType(FieldType type) {
        this.type = type;
   }
}

@Entity
public class FieldType extends PersistentEntity {
    ...
    @OneToMany(mappedBy="type", cascade=CascadeType.ALL)
    @IndexColumn(name="typeAttributeIndex")
    public List<FieldTypeAttribute> getAttributes() {
        return attributes;
    }
}

The generated SQL (show_sql=true):
Hibernate: insert into FieldTypeAttribute (name, fieldTypeAttribute_id, description, id) values (?, ?, ?, null)

HSQL Script:
create table FieldType (name varchar(255), description varchar(255), id integer generated by default as identity (start with 1), primary key (id))

create table FieldTypeAttribute (name varchar(255), description varchar(255), id integer generated by default as identity (start with 1), fieldTypeAttribute_id integer, typeAttributeIndex integer, primary key (id))

alter table FieldTypeAttribute add constraint FKDA86EDC8CB49B8FB foreign key (fieldTypeAttribute_id) references FieldType

INSERT INTO FIELDTYPE VALUES('Phone Number',NULL,1)

INSERT INTO FIELDTYPEATTRIBUTE VALUES('Area Code',NULL,1,1,NULL)

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cschoenfeld

Post subject: Workaround

Posted: Wed Feb 14, 2007 3:41 pm

Newbie

Joined: Fri Jan 26, 2007 5:52 am
Posts: 2

I stumbled across this today and found a solution.

It is possible to work around this problem by introducing a position property at the owned side of the association. The getPos() method always computes the position whereas setPos() is empty. You can use mappedBy as usual and an OrderBy annotation on the owner's collection getter. Thus, the position property is stored in the database in a write-only fashion and only used for sorting when the collection is loaded.

Implementation example:

Code:

@Entity
public class Parent {

    private List<Child> children;

    @OneToMany(cascade=CascadeType.ALL, mappedBy = "parent")
    @OrderBy(value = "pos")[
    public List<Child> getChildren() {
        return this.children;
    }

    public void setChildren(List<Child> children) {
        this.children = children;
    }

    int findChildPos(Child c) {
        int result = getChildren().indexOf(c);
        if (result == -1) {
            throw new IllegalStateException("Object hierarchy inconsistent. Child " 
              + c + " is not a child of Parent " + this);
        }
        return result;
    }
}

@Entity
public class Child {

    private Parent parent;

    @ManyToOne
    public Parent getParent() {
        return this.parent;
    }

    public void setParent(Parent parent) {
        this.parent = parent;
    }

    public Integer getPos() {
        if (this.parent == null) {
            return null;
        }
        return this.parent.findChildPos(this);
    }

    public void setPos(Integer pos) { 
        /* empty */
    }

}

Please note that this requires a correct implementation of hashCode() and equals() for the Child class in order for call to List.indexOf(Object) to work. But that should be a basic requirement in any case for persistent classes.

This workaround has proved to work with CascadeType.ALL when the order of the Parent.children list is modified and the Parent is updated. (At least {CascadeType.PERSIST, CascadeType.MERGE} are required for this, otherwise each Child has to be updated manually.)

Of course, this is not ideal as it is intrusive and pollutes domain model classes with technical trick code but it works.

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