I have a one-to-many relationship between Employee and Certificate

employee.hbm.xml
<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="hibernate.Employee" table="EMPLOYEE" >

<meta attribute="class-description">
This class contains the employee details.
</meta>

<id name="id" type="int" column="ID">
<generator class="sequence">
<param name="sequence">EMP_SEQ</param>
</generator>
</id>
<set name="certificates" cascade="all" table="CERTIFICATE" inverse="true" >
<key column="employee_id"/>
<one-to-many class="hibernate.Certificate" />
</set>

<property name="firstName" type="string" column="FIRST_NAME"/>
<property name="lastName" type="string" column="LAST_NAME"/>
<property name="salary" type="int" column="salary"/>

</class>
</hibernate-mapping>

Certificate.hbm.xml

<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="hibernate.Certificate" table="CERTIFICATE">
<meta attribute="class-description"> This class contains the certificate records. </meta>
<id name="id" type="int" column="ID">
<generator class="sequence">
<param name="sequence">cert_id</param>
</generator>
</id>

<property name="name" column="certificate_name" type="string"/>
<property name="empid" column="employee_id" type="int"/>
</class>
</hibernate-mapping>

hibernate.hbm.xml
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration SYSTEM "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory>
<property name="hibernate.dialect">
org.hibernate.dialect.Oracle10gDialect
</property>
<property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
<property name="hibernate.connection.url">
jdbc:oracle:thin:@localhost:1521:XE
</property>
<property name="hibernate.connection.username">aparna</property>
<property name="hibernate.connection.password">aparna1local</property>
<property name="hibernate.show_sql">true</property>
<!-- <property name="hibernate.current_session_context_class">thread</property> -->
<property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
<property name="hbm2ddl.auto">create</property>
<!-- <property name="hibernate.temp.use_jdbc_metadata_defaults">false</property> -->

<!-- List of XML mapping files -->
<mapping class="hibernate.Employee" resource="resources/Employee.hbm.xml"/>
<mapping class="hibernate.Certificate" resource="resources/Certificate.hbm.xml"/>

</session-factory>
</hibernate-configuration>


Though Sequence is generated correctly, insert Query for Certificate is picking up foreign_key as 0 and hence exception is thrown

17:39:23.609 [main] DEBUG org.hibernate.SQL - select EMP_SEQ.nextval from dual
Hibernate: select EMP_SEQ.nextval from dual
17:39:23.698 [main] DEBUG org.hibernate.id.SequenceGenerator - Sequence identifier generated: BasicHolder[java.lang.Integer[1]]
17:39:23.699 [main] DEBUG o.h.e.i.AbstractSaveEventListener - Generated identifier: 1, using strategy: org.hibernate.id.SequenceGenerator
17:39:23.781 [main] DEBUG org.hibernate.SQL - select cert_id.nextval from dual
Hibernate: select cert_id.nextval from dual
17:39:23.785 [main] DEBUG org.hibernate.id.SequenceGenerator - Sequence identifier generated: BasicHolder[java.lang.Integer[1]]
17:39:23.785 [main] DEBUG o.h.e.i.AbstractSaveEventListener - Generated identifier: 1, using strategy: org.hibernate.id.SequenceGenerator
17:39:23.786 [main] DEBUG org.hibernate.SQL - select cert_id.nextval from dual
Hibernate: select cert_id.nextval from dual
17:39:23.787 [main] DEBUG org.hibernate.id.SequenceGenerator - Sequence identifier generated: BasicHolder[java.lang.Integer[2]]
17:39:23.787 [main] DEBUG o.h.e.i.AbstractSaveEventListener - Generated identifier: 2, using strategy: org.hibernate.id.SequenceGenerator
17:39:23.788 [main] DEBUG org.hibernate.SQL - select cert_id.nextval from dual
Hibernate: select cert_id.nextval from dual
17:39:23.788 [main] DEBUG org.hibernate.id.SequenceGenerator - Sequence identifier generated: BasicHolder[java.lang.Integer[3]]
17:39:23.789 [main] DEBUG o.h.e.i.AbstractSaveEventListener - Generated identifier: 3, using strategy: org.hibernate.id.SequenceGenerator
17:39:23.789 [main] DEBUG o.h.e.t.spi.AbstractTransactionImpl - committing
17:39:23.790 [main] DEBUG o.h.e.i.AbstractFlushingEventListener - Processing flush-time cascades
17:39:23.792 [main] DEBUG o.h.e.i.AbstractFlushingEventListener - Dirty checking collections
17:39:23.797 [main] DEBUG o.h.engine.internal.Collections - Collection found: [hibernate.Employee.certificates#1], was: [<unreferenced>] (initialized)
17:39:23.813 [main] DEBUG o.h.e.i.AbstractFlushingEventListener - Flushed: 4 insertions, 0 updates, 0 deletions to 4 objects
17:39:23.814 [main] DEBUG o.h.e.i.AbstractFlushingEventListener - Flushed: 1 (re)creations, 0 updates, 0 removals to 1 collections
17:39:23.819 [main] DEBUG o.h.internal.util.EntityPrinter - Listing entities:
17:39:23.820 [main] DEBUG o.h.internal.util.EntityPrinter - hibernate.Certificate{empid=0, name=PMP, id=3}
17:39:23.820 [main] DEBUG o.h.internal.util.EntityPrinter - hibernate.Employee{firstName=Naik, lastName=AParna, certificates=[hibernate.Certificate#1, hibernate.Certificate#2, hibernate.Certificate#3], id=1, salary=30000}
17:39:23.820 [main] DEBUG o.h.internal.util.EntityPrinter - hibernate.Certificate{empid=0, name=MBA, id=1}
17:39:23.820 [main] DEBUG o.h.internal.util.EntityPrinter - hibernate.Certificate{empid=0, name=MCA, id=2}
17:39:23.871 [main] DEBUG org.hibernate.SQL - insert into EMPLOYEE (FIRST_NAME, LAST_NAME, salary, ID) values (?, ?, ?, ?)
Hibernate: insert into EMPLOYEE (FIRST_NAME, LAST_NAME, salary, ID) values (?, ?, ?, ?)
17:39:23.904 [main] DEBUG org.hibernate.SQL - insert into CERTIFICATE (certificate_name, employee_id, ID) values (?, ?, ?)
Hibernate: insert into CERTIFICATE (certificate_name, employee_id, ID) values (?, ?, ?)
17:39:23.940 [main] DEBUG o.h.e.jdbc.spi.SqlExceptionHelper - could not execute statement [n/a]
java.sql.SQLException: ORA-02291: integrity constraint (APARNA.FK_1T5PCA769LSFF735CNWKXBDQ4) violated - parent key not found

at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:112) ~[ojdbc14.jar:Oracle JDBC Driver version -

Setting this property in hibernate.cfg.xml created the table definition for Certificate table with Foreign Key as Not NULL and I guess Hibernate tried to put a value of 0 in the Employee ID field. I dropped the table, created it again with Employee ID (Foreign key) as NULL and deleted below entry. Its working fine now.
<property name="hbm2ddl.auto">create</property>