Question

I have a many to many (with a join table) relationship in my database that I need to model in hibernate.

I have POJO classes for all three tables in the database as the join table contains more than just the mapping information.

My problem occurs on save() of class A where class A is told to cascade to class AB (the join table) which is told to cascade to class B, but when it gets to the point where it is saving AB the foriegn key reference to table A is null.

At the end of the day I want to be able to call save() on an instance of the TableA class and have it save all three tables.

Table Diagram

TABLE A Mapping

<?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 Aug 27, 2012 2:16:37 PM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
    <class name="com.example.dbtables.TableA" table="Table A">
        <id name="id" type="int">
            <column name="id" />
            <generator class="sequence">
                <param name="sequence">table_a_seq</param>
            </generator>
        </id>
        <natural-id>
        <property name="data1" type="int">
            <column name="DATA1" not-null="true" />
        </property>
        <property name="data2" type="string">
            <column name="DATA2" length="16" not-null="true" />
        </property>
        </natural-id>
        <set name="tableAB" cascade="save-update" table="TableAB" inverse="true" lazy="true" fetch="select">
            <key>
                <column name="Table_AB_ID" not-null="true" />
            </key>
            <one-to-many class="com.example.dbtables.TableAB" />
        </set>
    </class>
</hibernate-mapping>

TABLE AB Mapping

<?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 Aug 27, 2012 2:16:37 PM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
    <class name="com.example.dbtables.TableAB" table="Table AB">
        <id name="id" type="int">
            <column name="id" />
            <generator class="sequence">
                <param name="sequence">table_ab_seq</param>
            </generator>
        </id>
        <natural-id>
        <many-to-one name="tableA" class="com.example.dbtables.TableA" fetch="select">
            <column name="Table_A_ID" not-null="true" />
        </many-to-one>
        <many-to-one name="tableB" cascade="save-update" class="com.example.dbtables.TableB" fetch="select">
            <column name="Table_B_ID" not-null="true" />
        </many-to-one>
        <property name="data1" type="int">
            <column name="DATA1" not-null="true" />
        </property>
            <property name="data2" type="int">
            <column name="DATA2" not-null="true" />
        </property>
        </natural-id>
    </class>
</hibernate-mapping>

TABLE B Mapping

<?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 Aug 27, 2012 2:16:37 PM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
    <class name="com.examples.dbtables.TableB" table="Table B">
        <id name="id" type="int">
            <column name="id" />
            <generator class="sequence">
                <param name="sequence">table_b_seq</param>
            </generator>
        </id>
        <natural-id>
        <property name="data1" type="string">
            <column name="DATA1" length="16" />
        </property>
        <property name="data2" type="java.lang.Integer">
            <column name="DATA2" />
        </property>
        </natural-id>
        <set name="TableAB" table="Table AB" inverse="true" lazy="true" fetch="select">
            <key>
                <column name="Table_AB_ID" not-null="true" />
            </key>
            <one-to-many class="com.example.dbtables.TableAB" />
        </set>
    </class>
</hibernate-mapping>

Thanks for any help you can give. Let me know if you need clarification or anymore information.

EDIT: My problem was that the references from AB -> to A were not getting correctly populated. This issue has been resolved thanks to everyone who helped.

Answer 1

  A a = new A();
  B b = new B();
  AB ab = new AB();
  ab.setA(a);
  ab.setB(b);
  a.setAB(ab);
  b.setAB(ab);

  save(a);  

This example not working? Did you get some exception?
EDIT
Look into this Hibernate doc Chapter 5. Basic O/R Mapping to chapter 5.1.4.1. Generator
PostgreSQL doesn’t support identity generation type.
Use sequence generation instead of identity, or something else