migration_v1.rb

def AddCounterCache < ActiveRecord::Migration
  def change
    add_column :users, :posts_count, :integer
  end
end

post.rb

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

user.html.erb

<ul>
  <% @users.each do |user| %>
  <li>
    <%= @user.name %> (<%= @user.posts.size %>)
  </li>
  <% end %>
</ul>

user.rb

def User < ActiveRecord::Base
  has_many :posts
end