N+1 Query là gì và Rails giải quyết bằng cách nào?

N+1 xảy ra khi load 1 collection rồi lặp để lấy association của mỗi item → 1 query lấy list + N query con.

# N+1: 1 query lấy posts + N query lấy user của mỗi post
Post.all.each { |p| puts p.user.name }

# FIX: includes eager-loads user cùng lúc
Post.includes(:user).each { |p| puts p.user.name }
# SQL: SELECT * FROM posts + SELECT * FROM users WHERE id IN (1, 2, ...)

includes vs eager_load vs preload:
- includes → Rails tự chọn strategy (preload hoặc eager_load).
- preload → luôn 2 query riêng.
- eager_load → luôn 1 query JOIN (dùng khi cần filter theo association).

Gem bullet tự phát hiện N+1 trong development.

N+1 occurs when loading a collection then iterating to fetch each item's association → 1 query for the list + N sub-queries.

# N+1: 1 query for posts + N queries for each post's user
Post.all.each { |p| puts p.user.name }

# FIX: includes eager-loads users in one go
Post.includes(:user).each { |p| puts p.user.name }
# SQL: SELECT * FROM posts + SELECT * FROM users WHERE id IN (1, 2, ...)

includes vs eager_load vs preload:
- includes → Rails picks the strategy automatically.
- preload → always 2 separate queries.
- eager_load → always 1 JOIN query (use when filtering on the association).

The bullet gem auto-detects N+1 in development.