Sharepoint 2013 custom search results grouping display template

Hi,

I am trying to implement custom grouping for search result items. I would like to group them by a managed property, but without using result blocks (query rules). The idea is to sort the results by the property and then compare the current item (ctx.CurrentItem) with the previous item using ctx.CurrentItem.ParentTableReference. If a new property value is detected we insert a new 'group header' (just some simple html). The problem is the ParentTableReference is undefined. Shouldn't there be such a property in current item object?
I'm implementing this in a custom search result item display template...

Thanks in advance.