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}