The groupby builtin filter currently do not support more that one dot.
For instance, if I have articles that I want to group by year, so I need to access item.date.year in the groupby filter, it is not possible by now, because jinja2 looks for the date.year attributes which do not exists.
This can probably be fixed by using operator.attrgetter in the filter.
groupby now supports attributes of attributes. This closed by 9573b66
groupby now supports attributes of attributes. This fixes #10