group by and map on values in group thanks @mattc

group_by_and_map.rb

# works in ruby 1.9
count.group_by { |o| o.site.name }.inject({}) { |m, (k,v)| m[k] = v.map(&:user_id); m }

# works in ruby 2.0 and above
count.group_by { |o| o.site.name }.map{ |k,v| [k, v.map(&:user_id)]}.to_h