getting the most recent row, grouped by an attribute's value - sql

Question

I'm working on a questionnaire for users to fill out. Users can answer each question many times. Responses have a question_key to pair them with their question. I want to get the full set of a user's current responses without retrieving every response created by a user.
The responses table looks like this:
create_table "responses", :force => true do |t|
t.string "question_key", :null => false
t.text "value", :null => false
t.integer "user_id", :null => false
t.datetime "created_at"
t.datetime "updated_at"
end
Here's the relevant part of the response class:
class Response < ActiveRecord::Base
def self.current
recent.uniq {|response| response.question_key}
end
scope :recent, order('created_at DESC')
end
user.responses.current gets the set of responses, but it seems really inefficient. It's getting all of the responses ever created by a user, and then throwing away everything but the most recent one for each unique value of question_key
I did figure out a SQL query that does what I want, ordering the rows before doing GROUP BY. Is there a reasonable way to use this in Response::current?
SELECT *
FROM (
SELECT *
FROM responses
WHERE user.id = 6
ORDER BY created_at DESC ) as user_responses
GROUP BY question_key

Answer 1

You can perform a group by in active record by using the group method.
Responses.where(user_id: '123').order('created_at DESC').group('question_key')
So your code would be something like:
class Response < ActiveRecord::Base
scope :recent, order('created_at DESC')
def self.current
recent.group('question_key')
end
end

Answer 2

This solution relies on the ids being sequential:
class Response < ActiveRecord::Base
def self.current
where(id: group(:question_key).maximum(:id).values)
end
end
It results in two queries when getting the current responses for a user (user.responses.current), but it avoids returning all of the user's responses.
SELECT MAX(`responses`.`id`) AS maximum_id, question_key AS question_key FROM `responses` WHERE `responses`.`user_id` = 6 GROUP BY question_key
SELECT `responses`.* FROM `responses` WHERE `responses`.`user_id` = 6 AND `responses`.`id` IN (452, 447, 9, 225, 190, 230, 240)