Many-to-one instead of one-to-one

N-te Jr. · **Posted:** Mon Apr 23, 2007 1:16 pm

Hello,

I've a problem and perhaps someone can help me.

I have two tables "Item" and "Molecule". There is a one-to-one relationship between them, but I get a many-to-one relationship created in the mapping file. Why?
(I designed the database schema with dbdesigner 4.)

So, if someone can help me that would be great.

Greetz Carina

Hibernate version: 3.2 beta 8

Name and version of the database you are using: Oracle 10g

Create Statement of the tables:

Code:

CREATE TABLE item (
  id INTEGER  NOT NULL ,
  type  VARCHAR(45) NULL,
  PRIMARY KEY(id)
);

CREATE TABLE molecules (
  item_id INTEGER  NOT NULL,
  name VARCHAR(100) NULL,
  smiles VARCHAR(100) NULL,
  iupac VARCHAR(100) NULL,
  inchi VARCHAR(100) NULL,
  formula VARCHAR(100) NULL,
  createBy VARCHAR(100) NULL,
  nofAtoms INT NULL,
  nofBonds INT NULL,
  nofHeavyAtoms INT NULL,
  nofHeavyBonds INT NULL,
  nofFragments INT NULL,
  nofSuperAtoms INT NULL,
  nofRings INT NULL,
  nofRGroups INT NULL,
  medianBondLength DOUBLE PRECISION NULL,
  mass DOUBLE PRECISION NULL,
  directory VARCHAR(100) NULL,
  workflowID INTEGER  NULL,
  PRIMARY KEY(item_id),
  FOREIGN KEY(item_id)
    REFERENCES item(id)
);

CREATE SEQUENCE item_seq 
start with 1 
increment by 1 
nomaxvalue;

CREATE TRIGGER item_trigger 
before insert on item 
for each row 
begin 
SELECT item_seq.nextval into :new.id from dual; 
end;
/

Mapping file:

Code:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated Apr 23, 2007 4:53:05 PM by Hibernate Tools 3.2.0.beta8 -->
<hibernate-mapping>
    <class name="de.fhg.scai.bio.csr.io.database.hibernate.Item" table="ITEM">
        <id name="id" type="int">
            <column name="ID" precision="22" scale="0" />
            <generator class="assigned" />
        </id>
        <property name="type" type="string">
            <column name="TYPE" length="45" />
        </property>
        <set name="moleculeses" inverse="true">
            <key>
                <column name="ITEM_ID" precision="22" scale="0" not-null="true" unique="true" />
            </key>
            <one-to-many class="de.fhg.scai.bio.csr.io.database.hibernate.Molecules" />
        </set>
    </class>
</hibernate-mapping>

cbredesen · **Posted:** Wed Apr 25, 2007 12:48 pm

Foreign key relationships in the database are modeled in hibernate as many-to-one. This is because relationally, a foreign key does not determine multiplicity. A foreign key on the related table means only that there is a relation; not that there is a unique relation. A FK defines only "-to-one", not "one" or "many".

So if you use one-to-one in Hibernate, you need to implement this with shared primary keys. Is suspect you don't want this, so what you need is a many-to-one with unique="true". This generates a unique constraint on the foreign key so the many-to-one has one-to-one multiplicity.

This is discussed thoroughly in Java Persistence with Hibernate :)

Hope this helps,

Chris

N-te Jr. · **Posted:** Thu Apr 26, 2007 5:53 am

So you say that I already have what I want to, am I right?!

cbredesen · **Posted:** Thu Apr 26, 2007 8:13 am

If what you intended was a FK relationship, then yes! :)

-Chris

N-te Jr. · **Posted:** Thu Apr 26, 2007 10:16 am

Thanks.

pranav · **Joined:** Thu May 03, 2007 4:20 pm **Posts:** 2

Hi
I have the same table structure.
If Item and Moleculeses were the VO and need to do a create in both tables.

How will the setMoleculeses method look?

public void setMoleculeses (Set moleculeses ) {
this.moleculeses = moleculeses ;

//Should this be opened and item set to this?

Iterator iterator =this.moleculeses .iterator();
if(iterator.hasNext()){

((Moleculeses )iterator.next()).setItem(this);
}

}
Thanks

cbredesen · **Posted:** Thu May 03, 2007 4:40 pm

I would make getMolecules/setMolecules protected so that only Hibernate uses them (or switch to field access). Then, write methods that are more like what you need:

Code:

/**
 * Add a molecule. Not used by Hibernate.
 */
public void addMolecule(Molecule m) {
  getMolecules().add(m);
  m.setItem(this);
}

and so on.

This keeps the domain model tight and works very well with Hibernate.

-Chris

pranav · **Joined:** Thu May 03, 2007 4:20 pm **Posts:** 2

Thank you!