Rails + Postgres query for all records that meet a criteria with a unique association ID

Question

I have some code like this:

stages.each do |stage|
  start_activity = stage.activities.where('created_at < ?', @start_time).first
  end_activity   = stage.activities.where('created_at < ?', @end_time).first
  # ...
end

Which fills my logs with a few screens worth of queries like these:

Activity Load (0.3ms)  SELECT  "activities".* FROM "activities"
  WHERE "activities"."stage_id" = $1
  AND (created_at < '2017-01-31 08:00:00.000000')
  ORDER BY id DESC, "activities"."created_at" DESC
  LIMIT 1 
  [["stage_id", 11548]]
Activity Load (0.3ms)  SELECT  "activities".* FROM "activities"
  ...

(Activity has a default scoped order of id DESC )

So I'm hoping there is a way to fetch all the activities I need per timestamp in a single query. This would reduce my number of from activity_count * 2 to simply 2 .

Here's what I tried, but I don't think distinct is properly being used here.

Activity
  .select('DISTINCT ON (stage_id) *')
  .where(stage: stages)
  .where('created_at < ?', @start_time)
  .reorder('id DESC')

Which spews out:

ActiveRecord::StatementInvalid (PG::InvalidColumnReference: ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions
LINE 1: SELECT DISTINCT ON (stage_id) * FROM "activities" WHERE "act...
                            ^
: SELECT DISTINCT ON (stage_id) * FROM "activities" WHERE "activities"."stage_id" IN (11548, 11549, <more ids snipped>, 12432, 12433) AND (created_at < '2017-01-31 08:00:00.000000')  ORDER BY id DESC):

What I mean to do here is to say: give me all activities that have a stage_id in an array of stages , where the created_at date is nearest to (without going over) a timestamp. What I want to get back is a single activity record for every stage in stages .

Is it possible to do this in 1 query rather than stages.count queries?

Answer 1

By taking a look at your error...

ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions

...I got to this Stack Overflow post .

You need to include 'account_id' in your order function somewhere.

So I think you should be able to do...

Activity
  .select('DISTINCT ON (stage_id)')
  .where(stage: stages)
  .where('created_at < ?', @start_time)
  .order(:stage_id, :created_at)