Rails应用程序中的Heroku PostgreSQL GROUP_BY错误

返回错误消息：

ActionView::Template::Error (PGError: ERROR:  
column "practices.id" must appear in the GROUP BY clause or be used in an aggregate function: 
SELECT "practices".* 
FROM "practices" WHERE ("practices".activity_id = 1) 
AND ("practices"."created_at" BETWEEN '2011-01-01' AND '2011-01-31') 
GROUP BY DATE(created_at) ORDER BY created_at DESC):

当我调用以下内容时会抛出此内容：

def month_days_not_practiced(date = Date.today)
   p = practices.where(:created_at => date.at_beginning_of_month..date.at_end_of_month).group("DATE(created_at)").to_a.count
   days_in_month(date.year,date.month) - p
end

我真的想保持代码干净,所以它适用于开发和生产数据库……任何人都可以解决这些问题吗？

我试过这个：

def month_days_not_practiced(date = Date.today)
   p = practices.where(:created_at => date.at_beginning_of_month..date.at_end_of_month).group("practices.id,DATE(created_at)").to_a.count
   days_in_month(date.year,date.month) - p
end

无济于事……

TIA.