How to use GroupTable

How to I use the GroupTable with entities an related entities?
I have an entity WorkOrder witch have a collection of Lines.
The lines as different attributes. Some is text, date and number.
When I try to group by WorkOrder it works.
When I try to add a second column witch comes from an nested datasource Lines
and I use line.createdDate or so I get an error
The first one was “can’t cast from entity to java List” the second was can’t convert from Integer to String.

So you display in one table the Order attributes and an attribute of the Line entity which is a collection inside Order? Does it make sense? What exact Line is displayed for an Order?

Hmm. I think I have to try this once more. I must have done something wrong.