Changing sort order with ActiveRecord find_in_batches#

Fri, 04 May 2012 07:29:34 +0000

The subject is Googlebait pure and simple, because the short version is that you can't. The very slightly longer and significantly more defensible version is that you may be able to but they don't want you to apparently because you might get the wrong answer if you mutate the data as you're traversing it (and because it's fast in MySql).

Personally I think the answer there is Well Don't Do That Then (and who cares about MySql) but that's just my opinion. If you want to order by, say, created_at descending, and perhaps you want to paginate the results, the only sensible conclusion to draw is that find_in_batches is just not intended for this use.

But it's not an unreasonable use. So I wrote ar-as-batches which lets you do

Users.where(country_id: 44).order(:joined_at).offset(200).as_batches do |user|
  user.party_all_night!
end

and it all works as you'd expect. I should of course caution you that

• you might get the wrong answer if you mutate the data as you're traversing it (so Don't Do That Then), and that
• ordering by something other than id ascending may be slower in MySql.

I don't know whether to be proud or ashamed of the tests , which check the generated queries by assigning a StringIO logger to ActiveRecord::Base.logger and then matching regexps in it after each test runs. There ought to be a better way. Perhaps there is a better way. Don't know what though.