问题
I have a Model which has two attributes (I am showing only two as only those two columns are needed)
MyModel
place_id ------------------------ user_id
1 ---------------------------------- 1
1 ---------------------------------- 2
1 ---------------------------------- 2
3 ---------------------------------- 3
3 ---------------------------------- 2
3 ---------------------------------- 3
3 ---------------------------------- 1
I want to fetch group by maximum records. Basically I want to fetch for a particular place which user has maximum records like for for place_id 1 user 2 has maximum results and for place 3 user 3 has maximum results.
回答1:
If you are specifying a place first you can do:
MyModel.where("place_id=?",place_id).
group("place_id,user_id").order("count(user_id) DESC").first
If you want to get all the maximums at once, then remove the where clause and iterate through your result set, taking only the first record for each distinct place_id. Or you may be able to add a distinct to the statement above. Something like this?
MyModel.select("DISTINCT(place_id)").
merge(MyModel.group("place_id,user_id").order("count(user_id) DESC"))
Good luck!
来源：https://stackoverflow.com/questions/6468316/maximum-in-group-by-active-record-query