获取第一条记录的列表中为每个组
我有一个登录模型下列：
["id", "username", "event", "parameters", "extras", "created_at", "updated_at"]
现在，我想获得的第一个日志每个用户名按 created_at 有序。
Now, I would like to get the first log for each username ordered by created_at.
要做到这一点的一种方式是运行为每个用户名以下查询：
One way to do this is to run the following query for each username:
log = Log.where("username = :username", username: username).order(:created_at).first
但是，这明显地查询数据库的很多倍（等于用户名的数量）。是否有某种方式做只有一个数据库查询？
But this obviously queries the database a lot of times (equal to the number of usernames). Is there some way to do only one database query?
为 DISTINCT ON另一种情况：
SELECT DISTINCT ON (username) *
FROM log
ORDER BY username, created_at;
给你的每个用户名。
详细信息：
Select在每一组中第一行按组？
Details:
Select first row in each GROUP BY group?
为Ruby / AR / Postgres的类似的回答：
Display用户从信息表，组最新消息
Similar answer for Ruby / AR / Postgres:
Display latest messages from messages table, group by user
如何执行原始SQL：
Table加入SQL到Rails活动记录查询
How to execute raw SQL:
Table join sql to rails active record query
我不是一个Ruby专家，但这个的Ruby语法应该工作：
I am not a Ruby expert, but this Ruby syntax should work:
Log.select("DISTINCT ON (username) *").order(:username, :created_at)