What is the N+1 query problem, and how does it commonly arise in a Node.js application using an ORM?

The N+1 query problem occurs when code fetches a list of N parent records with one query, then issues a separate additional query for each parent's related child data inside a loop, resulting in N+1 total queries instead of a single efficient join or batched query -- commonly introduced accidentally through an ORM's lazy-loading behavior, where accessing a related field triggers an implicit, easy-to-miss query.

// N+1 problem: one query for users, then one additional query per user for their orders
const users = await User.findAll();
for (const user of users) {
  const orders = await user.getOrders(); // triggers a separate query, N times
}

// Fixed: a single query with eager loading
const users = await User.findAll({ include: [Order] });