n-plus-one-killer

N+1 Killer

Find every N+1 in a Rails app and remove it. AI agents generate N+1s constantly — they write .each in a controller and call .author in the view without realizing each row triggers a fresh SELECT. This skill makes the agent detect first, then fix, then prevent.

Why this matters

The single most common Rails performance issue. A list endpoint with 20 rows and a hidden N+1 issues 21+ queries per request. At 100 RPS that's 2100 queries/second the DB didn't need to handle. The fix is one method call; the detection is what people miss.

The opinion (Rails 8 default)

Detect with Bullet in development + test, fail tests on N+1, fix with includes (or preload / eager_load when the trade-off is explicit). For the queries Bullet misses, add Prosopite. Reach for counter_cache and small denormalizations when the eager-loaded query is still too heavy.

Counter-position: gems like ar_lazy_preload and goldiloader automatically eager-load on first access in a loop. They eliminate the symptom but hide the cost — and they can over-eager-load (loading associations you only sometimes need). Use them only if the team has agreed to the trade-off; default to explicit eager loading.

Core patterns

Pattern 1: Detect with Bullet (dev + test)

Before (typical AI-generated controller, no detection in place):

class PostsController < ApplicationController
  def index
    @posts = Post.order(created_at: :desc).limit(20)
  end
end

Template:

<% @posts.each do |post| %>
  <%= post.title %> by <%= post.author.name %> (<%= post.comments.count %> comments)
<% end %>

This issues:

• 1 query for posts
• 20 queries for post.author (N+1 on belongs_to)
• 20 queries for post.comments.count (N+1 on counts)

Total: 41 queries.

After (Bullet wired up + fix applied):

# config/environments/development.rb
config.after_initialize do
  Bullet.enable        = true
  Bullet.alert         = false        # browser popup off
  Bullet.bullet_logger = true         # log/bullet.log
  Bullet.console       = true         # browser JS console
  Bullet.rails_logger  = true         # tail with `tail -f log/development.log`
  Bullet.add_footer    = true         # banner at the bottom of the page
end

# config/environments/test.rb
config.after_initialize do
  Bullet.enable = true
  Bullet.raise  = true                # fail the spec on any N+1
end