ruby 26 lines · 6 steps

Backfilling counter caches in a Rake task in Rails

A batched Rake task that recomputes stale Rails counter caches without hammering the database.

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Rails Intermediate counter-cache batching rake-tasks data-integrity throttling
1namespace :counter_cache do
2 desc "Recalculate comments_count for posts after a backfill"
3 task warm_post_comments: :environment do
4 scope = Post.where(comments_count: nil).or(Post.where("comments_count < 0"))
5 total = Post.count
6 warmed = 0
7 
8 Post.in_batches(of: 500).each_with_index do |batch, index|
9 batch.pluck(:id).each do |post_id|
10 Post.reset_counters(post_id, :comments, :likes)
11 warmed += 1
12 end
13 
14 progress = (warmed.to_f / total * 100).round(1)
15 Rails.logger.info("[counter_cache] batch #{index + 1}: warmed #{warmed}/#{total} (#{progress}%)")
16 sleep(0.1)
17 end
18 
19 stale = scope.count
20 if stale.positive?
21 warn "WARNING: #{stale} posts still have suspicious counts after warming"
22 else
23 puts "Counter caches warmed for #{warmed} posts"
24 end
25 end
26end
01 / 01
STEP 01

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Three takeaways
  1. 1reset_counters recomputes a counter cache from the real associated rows, fixing drift.
  2. 2Processing records in batches with a short sleep keeps a heavy backfill from overwhelming the database.
  3. 3Verifying with a suspicious-count query after the run turns a script into a self-checking migration.

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