Question

I have user with name, location, created_at as important fields in table.
I want to retrieve for each user the latest location,i.e, I want something like this:

username    location       created_at
abc         New York       2012-08-18 16:18:57
xyz         Mexico city    2012-08-18 16:18:57
abc         Atlanta        2012-08-11 16:18:57

only input is UId(1,2) array of userids.please help me to accomplish this.I just want to know how to write query using active record query interface.

Answer 1

Generally, this should be a standard way to solve this kind of problems:

SELECT l1.user, l1.location
FROM locations l1
LEFT JOIN locations l2 ON l1.user = l2.user AND l2.created_at > l1.created_at
WHERE l2.id IS NULL

The idea is to join the table with itself, and find those rows which don’t have any row with the same user and greater created_at.

Of course, you should have (user, created_at) index on your table.

Now you should see how would that be represented in AR interface.