rails-performance

Rails Performance Skill

Opinionated conventions for Rails performance. Every pattern has a clear unsafe anti-pattern and safe fix.

Sub-files:

• PROFILING.md — Profiling, benchmarking, and measurement workflows

---

N+1 Query Prevention

Always eager load associations accessed in loops:

@posts = Post.includes(:author)                        # single association
@post  = Post.includes(comments: :author).find(id)    # nested

Counter caches instead of .count in loops:

# Model: belongs_to :board, counter_cache: true
# Migration: add_column :boards, :cards_count, :integer, default: 0, null: false
board.cards_count   # reads column — no query
board.cards.count   # fires COUNT — N+1 in loops

---

Efficient Queries

# pluck returns plain arrays — no AR objects allocated
emails = User.where(active: true).pluck(:email)
ids    = Order.where(status: "pending").ids

# exists? → SELECT 1 LIMIT 1 (not .present?, .any?, .count > 0)
user.orders.where(status: "pending").exists?

# SQL aggregation — never load records to compute in Ruby
User.order(created_at: :desc)           # NOT .all.sort_by(&:created_at).reverse
Order.distinct.pluck(:status)           # NOT .all.map(&:status).uniq
Order.sum(:total_price)                 # NOT .all.sum(&:total_price)
Product.group(:category_id).count       # preload all counts in one query

Batch processing for large result sets:

User.find_each { |u| process(u) }                              # 1000 at a time
Order.where("created_at < ?", 1.year.ago).find_each(&:archive!)
Product.where(discontinued: true).find_in_batches(batch_size: 100) do |batch|
  Index.bulk_delete(batch)
end

---

Database Indexing

Index every FK column and every column used in where, order, find_by:

# t.references adds index by default
create_table :comments do |t|
  t.references :post, foreign_key: true
  t.references :user, foreign_key: true
end

add_index :articles, :status
add_index :articles, :slug, unique: true
add_index :users,    :email, where: "active = true", unique: true   # partial

# Composite for multi-column queries
add_index :orders, [:user_id, :status, :created_at]