N+1 Query (ARCH6050)

Key ruby.orm.n-plus-one-query · Severity warning · Category orm

Fetching a related record with a separate query for each element while iterating over a collection causes the N+1 query problem. This is the classic ORM performance killer.

Incorrect

posts = Post.all
posts.each do |post|
  puts post.author.name  # a separate SELECT per iteration
end

Correct

posts = Post.includes(:author)
posts.each do |post|
  puts post.author.name  # author loaded in one go
end

Fix

Eager-load related data with includes before the loop; do not issue a new query per iteration.