onFetch() not available for XML 'Join Data Set'

Hi,

I'm using Birt 2.6.2 with Javascript.

The data sitution is a classical one to many relation with 3 tables.

SYSTEM <- SYSTEMCOMPONENT -> COMPONENTS

A system with some attributes and many components with their attributes assigned to a system.

In my report I need to use code inside onFetch() of my ODA Data Set.

Having an SQL DB as datasource I can put all JOINS inside my SQL query and have only one Dataset with all my data.
For that one Dataset I can use code inside onFetch().

Having XML as datasource, I have to map all elements in different 'Data Set' Elements
and have to use 'Join Data Set' Elements to bring everything together in one (Join)Data Set Element like in SQL.
But for 'Join Data Set' Elements there are no scripts like beforeOpen() or onFetch() available.
In the script tab is to see 'This element has no methods'.

If I need such a 'Join Data Set' Element just as a binding source for a grid, there is no problem.

But if I want to script over all data I can't use just one of the single 'Data Set' Elements.
I need the object that contains all elements, but that is not possible.

How can I use onFetch() if the datasource is XML and I have to use 'Join Data Set' Elements ?

regards
germobirt