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skills/rails-performance-patterns/skill.md

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+---
+name: rails-performance-patterns
+description: Detects N+1 queries, suggests eager loading, and recommends database indexes
+auto_invoke: true
+trigger_on: [file_modify]
+file_patterns: ["**/models/**/*.rb", "**/controllers/**/*.rb"]
+tags: [rails, performance, optimization, n+1, eager-loading, indexes]
+priority: 2
+version: 2.0
+---
+
+# Rails Performance Patterns Skill
+
+Automatically detects performance issues and suggests optimizations.
+
+## What This Skill Does
+
+**Automatic Detection:**
+- N+1 query problems (missing eager loading)
+- Missing database indexes on foreign keys
+- Inefficient query patterns
+- Large result sets without pagination
+
+**When It Activates:**
+- Model files with associations modified
+- Controller actions that query models
+- Iteration over associations detected
+
+## Key Checks
+
+### 1. N+1 Query Detection
+
+**Problem Pattern:**
+```ruby
+# app/controllers/posts_controller.rb
+def index
+  @posts = Post.all  # 1 query
+  @posts.each do |post|
+    puts post.author.name  # N queries (one per post)
+  end
+end
+```
+
+**Skill Output:**
+```
+⚠️  Performance: Potential N+1 query
+Location: app/controllers/posts_controller.rb:15
+Issue: Accessing 'author' association in loop without eager loading
+
+Fix: Add includes to eager load:
+@posts = Post.includes(:author).all
+```
+
+**Solution:**
+```ruby
+def index
+  @posts = Post.includes(:author).all  # 2 queries total
+end
+```
+
+### 2. Missing Indexes
+
+**Checks:**
+- Foreign keys have indexes
+- Commonly queried columns indexed
+- Unique constraints have indexes
+
+**Example:**
+```ruby
+# db/migrate/xxx_create_posts.rb
+create_table :posts do |t|
+  t.references :user  # ✅ Auto-creates index
+  t.string :slug      # ❌ Missing index if queried often
+end
+```
+
+**Skill Output:**
+```
+⚠️  Performance: Missing index recommendation
+Location: app/models/post.rb:5
+Issue: slug column used in where clauses without index
+
+Add migration:
+add_index :posts, :slug, unique: true
+```
+
+### 3. Pagination Missing
+
+**Problem:**
+```ruby
+def index
+  @products = Product.all  # ❌ Loads all 100k+ products
+end
+```
+
+**Skill Output:**
+```
+⚠️  Performance: Large result set without pagination
+Location: app/controllers/products_controller.rb:10
+Issue: Loading all Product records (estimated 100k+ rows)
+
+Recommendation: Add pagination
+# Use kaminari or pagy
+@products = Product.page(params[:page]).per(20)
+```
+
+### 4. Counter Cache Opportunities
+
+**Pattern:**
+```ruby
+# Without counter cache
+@user.posts.count  # Runs COUNT(*) query every time
+
+# With counter cache
+@user.posts_count  # Reads from cached column
+```
+
+**Skill Output:**
+```
+ℹ️  Performance: Counter cache opportunity
+Location: app/views/users/show.html.erb:12
+Pattern: Frequently accessing post count
+
+Add to migration:
+add_column :users, :posts_count, :integer, default: 0
+
+Update association:
+belongs_to :user, counter_cache: true
+```
+
+## Configuration
+
+```yaml
+# .rails-performance.yml
+n1_detection:
+  enabled: true
+  severity: warning
+
+indexes:
+  check_foreign_keys: true
+  check_query_columns: true
+
+pagination:
+  warn_threshold: 1000
+  require_for_large_tables: true
+
+counter_cache:
+  suggest_threshold: 3  # Suggest if accessed 3+ times
+```
+
+## Monitoring Integration
+
+**Add instrumentation:**
+```ruby
+# config/initializers/query_monitoring.rb
+ActiveSupport::Notifications.subscribe('sql.active_record') do |*args|
+  event = ActiveSupport::Notifications::Event.new(*args)
+  if event.duration > 100  # Log slow queries
+    Rails.logger.warn("Slow query (#{event.duration}ms): #{event.payload[:sql]}")
+  end
+end
+```
+
+## Common Optimizations
+
+**Eager Loading:**
+```ruby
+# N+1
+Post.all.each { |p| p.author.name }
+
+# Fixed: includes (left join)
+Post.includes(:author).each { |p| p.author.name }
+
+# Fixed: preload (separate queries)
+Post.preload(:author).each { |p| p.author.name }
+
+# Fixed: eager_load (always joins)
+Post.eager_load(:author).each { |p| p.author.name }
+```
+
+**Select Specific Columns:**
+```ruby
+# Loads all columns
+Post.all
+
+# Loads only needed columns
+Post.select(:id, :title, :created_at)
+```
+
+**Batch Processing:**
+```ruby
+# Loads all at once
+Post.all.each { |p| process(p) }
+
+# Loads in batches of 1000
+Post.find_each(batch_size: 1000) { |p| process(p) }
+```
+
+## References
+
+- **Rails Performance Guide**: https://guides.rubyonrails.org/performance_testing.html
+- **Bullet Gem**: https://github.com/flyerhzm/bullet
+- **Pattern Library**: /patterns/caching-patterns.md
+
+---
+
+**This skill helps you build fast Rails applications from the start.**