An n+1 queries problem means that a query is executed for every result of a previous query.

For example, with this records…

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

If you execute the next code…

Post.all.each do |post|
  post.comments.each do |comment|
    puts comment.body
  end
end

You will execute…

• 1 query to fetch the posts (Post.all)
• And n queries to fetch the comments of each post (post.comments)

If there are…

• 2 posts, it will execute 3 queries (2 + 1)
• 5 posts, it will execute 6 queries (5 + 1)
• 1000 posts, it will execute 1001 queries (1000 + 1)

Related articles

• When is better to use preload or eager_load vs includes?
• Solve n+1 queries and slow counts with counter caches
• A tool and examples to help you implement the first or last per record in rails with one query
• When do you need a counter cache in rails?