example.rb

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user, counter_cache: true
end

user = User.create
user.posts_count # => 0
user.posts << Post.new # executes `UPDATE users SET posts_count = COALESCE(posts_count, 0) + 1 WHERE id = ?`
user.posts_count # => 0
user.reload.posts_count # => 1