illegal try associate collection twin open sessions

i'm perplexing supplement pojo collection another pojo. i'm certain i'm origination unequivocally ridiculous mistake somewhere along lines nonetheless i can't figure out solve it.

i have pojo lookuptable contains list columns:

public category lookuptable {
  private enlarged id;
  // some-more properties here...
  private list<column> columns;

  open vacant addcolumn(column column) {
    this.columns.add(column);
  }

  // some-more methods here...
}

in hibernate settlement i have:

<class name="lookuptable" table="arr_lookup_table">
  <id name="id" column="id">
    <generator class="native"/>
  </id>
  <!-- properties here -->
  <bag name="columns" cascade="all,delete-orphan" access="field">
    <key column="lookup_table" not-null="true"/>
    <one-to-many class="column"/>
  </bag>
</class>

<class name="column" table="arr_lookup_column">
  <id name="id" column="id">
    <generator class="native"/>
  </id>
  <!-- properties here -->
</class>

in open config record i have:

<tx:advice id="txadvice" transaction-manager="txmanager">
  <tx:attributes>
    <tx:method name="*" propagation="required"/>
  </tx:attributes>
</tx:advice>
<aop:config>
  <aop:pointcut id="managers" expression="execution(public * com.foo.*manager.*(..))"/>
  <aop:advisor advice-ref="txadvice" pointcut-ref="managers"/>
</aop:config>

and finally formula where fails within manager category (com.foo.lookuptablemanager):

public vacant addcolumntotable(column column, enlarged tableid) {
  lookuptable lookuptable = this.lookuptabledao.findbyid(tableid);
  lookuptable.addcolumn(column);
  this.lookuptabledao.saveorupdate(lookuptable);
}

the non-static lookuptabledao here refers rudimentary dao category extends hibernatedaosupport.

the blunder i is:

org.hibernate.hibernateexception: illicit try associate collection twin open sessions
  during org.hibernate.collection.abstractpersistentcollection.setcurrentsession(abstractpersistentcollection.java:410)
  during org.hibernate.event.def.onupdatevisitor.processcollection(onupdatevisitor.java:43)
  during org.hibernate.event.def.abstractvisitor.processvalue(abstractvisitor.java:101)
  during org.hibernate.event.def.abstractvisitor.processvalue(abstractvisitor.java:61)
  during org.hibernate.event.def.abstractvisitor.processentitypropertyvalues(abstractvisitor.java:55)
  during org.hibernate.event.def.abstractvisitor.process(abstractvisitor.java:123)
  during org.hibernate.event.def.defaultsaveorupdateeventlistener.performupdate(defaultsaveorupdateeventlistener.java:293)
  during org.hibernate.event.def.defaultsaveorupdateeventlistener.entityisdetached(defaultsaveorupdateeventlistener.java:223)
  during org.hibernate.event.def.defaultsaveorupdateeventlistener.performsaveorupdate(defaultsaveorupdateeventlistener.java:89)
  during org.hibernate.event.def.defaultsaveorupdateeventlistener.onsaveorupdate(defaultsaveorupdateeventlistener.java:70)
  during org.hibernate.impl.sessionimpl.firesaveorupdate(sessionimpl.java:507)
  during org.hibernate.impl.sessionimpl.saveorupdate(sessionimpl.java:499)
  during org.hibernate.impl.sessionimpl.saveorupdate(sessionimpl.java:495)
  during com.foo.abstractdao.saveorupdate(abstractdao.java:29)
  during com.foo.lookuptablemanager.addcolumntotable(lookuptablemanager.java:338)
... etc ...

ok, i know simple summary i'm getting. nonetheless i don't know where i second session.... anyone assistance me this?

i'm controlling hibernate 3.2.6.ga, open 2.5.5 tomcat 6.0