Get top level work item parent on RRDI cognos report

As we are in phase of migrating all our BIRT reports to Cognos RRDI reports, I have some questions to ask  for advice.

We have some complex report definitions in BIRT that we don't know in this moment how to migrate them to Cognos reports.

I 've alredy migrated some reports, but few of them are still in progress.

Reports about I am talking to are parent-child related work items.

I figured out that there is existing table Request related requests (by relational link) which gives parent work item, but only one level. I would like to get top lever parent work item of any child in work item hierarchy structure and show it on report.

My question is how and from where to start to build such kind of report in Cognos?

Are there any good materials for this on jazz.net library or wiki?

Thanks,