Railsで日付で集計する

http://qiita.com/ryonext/items/4ed2f7635135a7b388f3
DATE(:created_at)が使える

pry(main)> Deal.all.group("date(created_at)").select("date(created_at)").count
   (0.5ms)  SELECT COUNT(date(created_at)) AS count_date_created_at, date(created_at) AS date_created_at FROM "deals" GROUP BY date(created_at)
=> {Fri, 29 May 2015=>2}