Converting sql query to ActiveRecord Rails - mysql

Question

I have a location table in my database which contains location data of all the users of my system.
The table design is something like
id| user_id| longitude| latitude| created_at|
I have an array of users. Now I want to select the latest(sorted according to created at) location of all these users.
I am able to figure out the sql query for same
SELECT * FROM my_table
WHERE (user_id , created_at) IN (
SELECT user_id, MAX(created_at)
FROM my_table
GROUP BY user_id
)
AND user_id IN ('user1', 'user2', ... );
Now as I am working in Ruby On Rails, I want to write this sql query to activerecord rails. Can anyone please help me with this ?

Answer 1

I think this will give the correct result:
MyModel.order(created_at: :desc).group(:user_id).distinct(:user_id)
If you want to generate the exact same query, this will do it:
MyModel.where("(user_id, created_at) IN (SELECT user_id, MAX(created_at) from my_table GROUP BY user_id)")
I think the subquery will probably not scale well with a large data set, but I understand if you just want to get it into rails and optimize later.

Answer 2

How about adding a scope, and getting the same result in a slightly different way:
class UserLocation
def self.latest_per_user
where("user_locations.created_at = (select Max(ul2.created_at) from user_locations ul2 where ul2.user_id = user_locations.user_id)")
end
end
Then you just use:
UserLocation.latest_per_user.where(:user_id => ['user1', 'user2'])
... to get the required data set.