javax.faces.FacesException: javax.ejb.EJBException: nested exception is: c2m2.api.PersistenceException: org.hibernate.HibernateException: Illegal attempt to associate a collection with two open sessions
   at ch.centrisag.c2m2.webui.delegate.AdminDelegate.deleteMaillist(AdminDelegate.java:285)
   at ch.centrisag.c2m2.webui.control.notification.MaillistQueryAction.delete(MaillistQueryAction.java:67)
   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 com.sun.el.parser.AstValue.invoke(Unknown Source)
   at com.sun.el.MethodExpressionImpl.invoke(Unknown Source)
   at com.sun.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:68)
   at javax.faces.component._MethodExpressionToMethodBinding.invoke(_MethodExpressionToMethodBinding.java:75)
   at org.apache.myfaces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:54)
   at javax.faces.component.UICommand.broadcast(UICommand.java:121)
   at org.ajax4jsf.component.UIDataAdaptor.broadcast(UIDataAdaptor.java:1359)
   at org.ajax4jsf.component.AjaxViewRoot.processEvents(AjaxViewRoot.java:317)
   at org.ajax4jsf.component.AjaxViewRoot.broadcastEvents(AjaxViewRoot.java:292)
   at org.ajax4jsf.component.AjaxViewRoot.processPhase(AjaxViewRoot.java:249)
   at org.ajax4jsf.component.AjaxViewRoot.processApplication(AjaxViewRoot.java:462)
   at org.apache.myfaces.lifecycle.InvokeApplicationExecutor.execute(InvokeApplicationExecutor.java:32)
   at org.apache.myfaces.lifecycle.LifecycleImpl.executePhase(LifecycleImpl.java:103)
   at org.apache.myfaces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:76)
   at javax.faces.webapp.FacesServlet.service(FacesServlet.java:148)
   at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:225)
   at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:127)
   at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:283)
   at weblogic.servlet.internal.TailFilter.doFilter(TailFilter.java:26)
   at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:42)
   at org.ajax4jsf.webapp.BaseXMLFilter.doXmlFilter(BaseXMLFilter.java:154)
   at org.ajax4jsf.webapp.BaseFilter.handleRequest(BaseFilter.java:260)
   at org.ajax4jsf.webapp.BaseFilter.processUploadsAndHandleRequest(BaseFilter.java:366)
   at org.ajax4jsf.webapp.BaseFilter.doFilter(BaseFilter.java:493)
   at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:42)
   at org.apache.myfaces.webapp.filter.ExtensionsFilter.doFilter(ExtensionsFilter.java:147)
   at weblogic.servlet.internal.FilterChainImpl.doFilter(FilterChainImpl.java:42)
   at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3212)
   at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
   at weblogic.security.service.SecurityManager.runAs(SecurityManager.java:121)
   at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:1983)
   at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:1890)
   at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1344)
   at weblogic.work.ExecuteThread.execute(ExecuteThread.java:209)
   at weblogic.work.ExecuteThread.run(ExecuteThread.java:181)
Caused by: javax.ejb.EJBException: nested exception is: c2m2.api.PersistenceException: org.hibernate.HibernateException: Illegal attempt to associate a collection with two open sessions
   at c2m2.ejb.AdminServicesBean.deleteMaillist(AdminServicesBean.java:496)
   at c2m2.ejb.AdminServicesEJB_1iookz_ELOImpl.deleteMaillist(AdminServicesEJB_1iookz_ELOImpl.java:81)
   at c2m2.webui.delegate.AdminDelegate.deleteMaillist(AdminDelegate.java:281)

@Entity
public class MaillistAssociation extends BaseEntity {
   
   @Id
   @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SEQ_BO_MAILLIST")
   @Column(name="ID")
   private Long id;
   
   @ManyToOne
   @JoinColumn(name = "MAILLIST_ID")
   private Maillist maillist;
.
.
.

}


@Entity
@Table(name="CM_MAILLIST")
@SequenceGenerator(name="SEQ_MAILLIST", sequenceName="SEQ_CM_MAILLIST")
public class Maillist extends BaseEntity {


   @Id
   @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="SEQ_MAILLIST")
   @Column(name="ID")
   private Long id;
   
   @Basic
   @Column(name="NAME")
   private String name;
   
   @ManyToMany(
         fetch=FetchType.EAGER,
         cascade={CascadeType.ALL}
   )
   @JoinTable(
         name = "CM_MAILLIST_EMAIL",
         joinColumns = {@JoinColumn(name = "MAILLIST_ID")},
         inverseJoinColumns = {@JoinColumn(name = "EMAIL_ID")}
   )
   private Set<Email> emails = new HashSet<Email>();
   
   @OneToMany(mappedBy="maillist")
   private Set<MaillistAssociation> maillistAssociations = new HashSet<MaillistAssociation>();
.
.
.
}