Not sure if this solution will work for you but this is what I did to
implement nested grouping using SOLR 3.X.
Simple idea behind is to Concatenate 2 fields and index them in to single
field and group on that field..
http://stackoverflow.com/questions/12202023/field-collapsing-grouping-how-to-ma
There may be a terminology problem here. In Solr land, "grouping"
aka "field collapsing" governs how the results are returned. But
from your example, it looks like you really want summary counts
rather than return documents grouped by some field.
If you want counting, take a look at "pivot facetin
