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3. MarkUs: Performance

My first impactful bug fix—instructors notice this one.

PR #7678

The Problem

A page was making 200+ SQL queries when it should have made 1. Classic N+1 pattern: loading a list, then lazily loading an association for each item.

# Before: N+1 queries
students.each do |student|
  puts student.user.name  # Each access triggers a query
end

The Fix

# After: 1 query with eager loading
students.includes(:user).each do |student|
  puts student.user.name  # Already loaded
end

Added includes(:user) to fix the 200→1 SQL query explosion.

What It Taught Me

Spotting N+1 patterns — Any time you loop over records and access an association, you might have an N+1. Watch for:

  • Loops in views
  • Serializers that access associations
  • Any each block touching related models

Rails eager loading — includes, preload, and eager_load each have different behaviors. Know when to use which.

Academic platform performance — When 50 TAs grade simultaneously, every query counts. What’s fine for 10 users breaks at 100.

This was my first “impactful” fix. Instructors actually noticed the page load time improvement.