The JSON Facet API currently has the ability to map between parents and children ( see http://yonik.com/solr-nested-objects/ )
This issue is about adding a true rollup ability where parents would take on derived values from their children. The most important part (and the most difficult part) will be the external API.
The bottom line is to introduce uniqueBlock(_root_) aggregation, which is supposed to be faster than unique(_root_) and purposed for blocked index only. For now it it supports singlevalue string fields, docValues usually make sense.