Why this simple hibernate sample doesn't work?

Question

I wrote these two classes:

public class ClasseA { Integer id; String numero; ClasseB cb; public ClasseB getCb() { return cb; } public void setCb(ClasseB cb) { this.cb = cb; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getNumero() { return numero; } public void setNumero(String numero) { this.numero = numero; } }

and

public class ClasseB { Integer id; String annotazione; public String getAnnotazione() { return annotazione; } public void setAnnotazione(String annotazione) { this.annotazione = annotazione; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } }

As you can see in ClasseA there is a reference to ClasseB.

this is the mapping:

 <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class dynamic-insert="false" dynamic-update="false" mutable="true" name="ClasseA" optimistic-lock="version" polymorphism="implicit" select-before-update="false"> <id name="id" type="java.lang.Integer"> <column name="id"/> <generator class="identity"/> </id> <property name="numero" type="java.lang.String"> <column name="numero"/> </property> <one-to-one cascade="all" class="ClasseB" name="cb"/> </class> <class dynamic-insert="false" dynamic-update="false" mutable="true" name="ClasseB" optimistic-lock="version" polymorphism="implicit" select-before-update="false"> <id name="id" type="java.lang.Integer"> <column name="id"/> <generator class="identity"/> </id> <property name="annotazione" type="java.lang.String"> <column name="annotazione"/> </property> </class> </hibernate-mapping>

Two things don't work as expected:

First of all, since I'm using hdb2ddl with update I'd expect do generate a table for classeA with a reference to classeB (and of course a table for classeB). Which isn't. All I get is:

CREATE TABLE classea ( id INT(11) NOT NULL AUTO_INCREMENT, numero VARCHAR(255) DEFAULT NULL,
PRIMARY KEY (id) ) ENGINE=INNODB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1

CREATE TABLE classeb ( id int(11) NOT NULL AUTO_INCREMENT, annotazione varchar(255) DEFAULT NULL, PRIMARY KEY (id) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=latin1
Second: If I save an istance of classeA with cb correctly set to a cb istance, it will work putting a row on the first and a row on the second table. But on retrivial it doesn't even load classeA....

Please help as I think I didn't understand properly this kind of association. Please don't suggest to use annotations as I can't. Tnx in advance.

Instantsoup · Accepted Answer · 2010-02-10 16:55:59Z

My understanding with one-to-one is that they share identity. That is, when you create a ClasseA, the ClasseB inside of ClasseA will have the same id. There is no explicit reference between the two tables.

For sanity, I often use many-to-one, even in the one-to-one scenario, because this creates the foreign key column in the database like you are expecting.

I'm not sure about the retrieval issue you are having without a little more information about the code being used to save and load.

user271287 · Accepted Answer · 2010-02-12 21:00:35Z

I think the problem is the Class B's id isn't properly associated to Class A. I assume you're associating the 2 classes by primary key. Then try this.

First, for one-to-one relationship, both classes need one-to-one mapping configured.

Add this to Class B's definition (note: The constrained attribute is important to the hibernate schema generation tool.):

<one-to-one class="ClasseA" name="ca" constrained="true" />

Second, making sure your dependent class associate its id to Class A by using

<generator class="foreign">

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