Hi, I have 2 classes:

public class A implements Serializable(){
private Integer id;
private Integer foreignId;
A(){}
public void setId(Integer id){this.id=id;}
public Integer getId(){return this.id;}
public void setForeignId(Integer id){this.foreignId=id;}
public Integer getForeignId(){return this.foreignId;}
}

public class B implements Serializable(){
private Integer id;
private Integer something;
B(){}
public void setId(Integer id){this.id=id;}
public Integer getId(){return this.id;}
public void setSomething(Integer s){this.something=s;}
public Integer getSomething(){return this.something;}
}

and mappings

<hibernate-mapping>
<class name="A" table="a">
<id name="id">
<generator class="sequence"/>
</id>
<many-to-one name="foreignId" insert="true" class="B" />
</hibernate-mapping>

<hibernate-mapping>
<class name="B" table="b">
<id name="id">
<generator class="sequence"/>
</id>
<property name="something"/>
</hibernate-mapping>

in table b, i have the raw |1|2|
and then

tx=session.beginTransaction();
A a=new A();
a.setForeignId(new Integer(1)); //from table b
session.save(a);
tx.commit();

after commit, the exception is thrown

Exception in thread "main" java.lang.RuntimeException: org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of B.id
at newB(MyUtil.java:350)
at Main.main(Main.java:45)
Caused by: org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of B.id
at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:171)
at org.hibernate.tuple.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:176)
at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:3257)
at org.hibernate.persister.entity.AbstractEntityPersister.isTransient(AbstractEntityPersister.java:2983)
at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:181)
at org.hibernate.engine.ForeignKeys$Nullifier.isNullifiable(ForeignKeys.java:137)
at org.hibernate.engine.ForeignKeys$Nullifier.nullifyTransientReferences(ForeignKeys.java:69)
at org.hibernate.engine.ForeignKeys$Nullifier.nullifyTransientReferences(ForeignKeys.java:47)
at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:263)
at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:167)
at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:114)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:186)
at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:175)
at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:557)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:545)
at org.hibernate.impl.SessionImpl.save(SessionImpl.java:541)
at cz.muni.fi.xsubcik.lis.session.Librarian.newItem(Librarian.java:345)
... 1 more
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:145)
... 20 more
Java Result: 1

what i am doing wrong?

(Just in case you're not watching the other thread anymore: http://forum.hibernate.org/viewtopic.php?p=2286935#2286935)

-- Jim