Optimize impressionist_count query via arel count with distinct option

app/models/impressionist/impressionable.rb

@@ -26,10 +26,7 @@ module Impressionist
     def impressionist_count(options={})
       options.reverse_merge!(:filter=>:request_hash, :start_date=>nil, :end_date=>Time.now)
       imps = options[:start_date].blank? ? impressions : impressions.where("created_at>=? and created_at<=?",options[:start_date],options[:end_date])
-      if options[:filter]!=:all
-        imps = imps.select(options[:filter]).group(options[:filter])
-      end
-      imps.all.size
+      options[:filter] == :all ? imps.count : imps.count(options[:filter], :distinct => true)
     end
 
     def update_counter_cache