ruby 22 lines · 7 steps

How to backfill a column safely in Rails

A batched migration fills a new full_name column without locking the whole users table.

Explained by highlit
Rails Intermediate migrations batching backfill database zero-downtime
1class BackfillUsersFullName < ActiveRecord::Migration[7.1]
2 disable_ddl_transaction!
3 
4 BATCH_SIZE = 5_000
5 
6 class User < ActiveRecord::Base
7 self.table_name = :users
8 end
9 
10 def up
11 User.unscoped.where(full_name: nil).in_batches(of: BATCH_SIZE) do |relation|
12 relation.update_all("full_name = TRIM(CONCAT(first_name, ' ', last_name))")
13 sleep(0.05)
14 end
15 end
16 
17 def down
18 User.unscoped.where.not(full_name: nil).in_batches(of: BATCH_SIZE) do |relation|
19 relation.update_all(full_name: nil)
20 end
21 end
22end
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Backfilling large tables in bounded batches keeps locks short and avoids blocking production traffic.
  2. 2Defining a lightweight model inside the migration insulates it from later changes to the real class.
  3. 3A reversible migration pairs its up-transformation with a down that returns the column to its prior state.

Related explainers

ruby 
require "shellwords"
require "open3"
 
module Backup

Building safe shell commands in Ruby

shell-out subprocess command-injection
Intermediate 7 steps
ruby 
class UserAgentParser
  BROWSERS = [
    [/Edg\/([\d.]+)/, "Edge"],
    [/OPR\/([\d.]+)/, "Opera"],

Parsing user-agent strings in Ruby

regex pattern-matching lookup-tables
Intermediate 8 steps
ruby 
class LogAggregator
  BUCKET_FORMAT = "%Y-%m-%dT%H:%M"
 
  def initialize(entries)

Bucketing log entries by the minute in Ruby

aggregation hashing enumerable
Intermediate 5 steps
ruby 
class WeeklySignupsReport
  DEFAULT_WEEKS = 12
 
  def initialize(weeks: DEFAULT_WEEKS, source: User.all)

Building a weekly signups report in Rails

service object aggregation group by
Intermediate 7 steps
ruby 
class ApplicationController < ActionController::Base
  EXPERIMENTS = {
    checkout_button_color: %w[control blue green],
    onboarding_flow: %w[control streamlined]

How A/B test cohorts are assigned in Rails

a-b-testing cookies hashing
Intermediate 8 steps
ruby 
class SnippetHighlighter
  CONTEXT_RADIUS = 60
  MAX_TERMS = 8
 

Building search-result snippets in Ruby

regular-expressions text-processing search
Intermediate 9 steps

Share this explainer

Here's the card — post it anywhere.

How to backfill a column safely in Rails — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code