计算每月的推荐的合格人人数

users = User.where(:nickname.in => ["橙子","柏图卡","1circle","jensen鑫","老猴走四方","琳子想要跑着玩","神叨M"])
a = []
users.each do |user|
  b = 0
  user.referring.each do |r|
    if ("2014-01-01 00:00:00".to_datetime.."2014-01-31 23:59:59".to_datetime).cover?(r.created_at)
      if r.referred?
        b += 1
      end
    end
  end
  a << { user.nickname => b }
end