:counter_cache finally working

20121112152734_create_users.rb

# db/migrate/20121112152734_create_users.rb
class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name
      t.integer :groups_count, :default => 0

      t.timestamps
    end
  end
end

20121112152847_create_user_groups.rb

# db/migrate/20121112152847_create_user_groups.rb
class CreateUserGroups < ActiveRecord::Migration
  def change
    create_table :user_groups do |t|
      t.string :name
      t.integer :user_id

      t.timestamps
    end
  end
end

gistfile1.rb

# From the console I run

$ rails console
1.9.3p194 :001 > user = User.create :name => "joe"
   (0.0ms)  begin transaction
  SQL (3.2ms)  INSERT INTO "users" ("created_at", "groups_count", "name", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Mon, 12 Nov 2012 18:57:54 UTC +00:00], ["groups_count", 0], ["name", "joe"], ["updated_at", Mon, 12 Nov 2012 18:57:54 UTC +00:00]]
   (12.0ms)  commit transaction
 => #<User id: 3, name: "joe", groups_count: 0, created_at: "2012-11-12 18:57:54", updated_at: "2012-11-12 18:57:54">
1.9.3p194 :002 > user.groups.size
 => 0
1.9.3p194 :003 >

# Now the size method does not execute an SQL query

user.rb

# app/models/user.rb
class User < ActiveRecord::Base
  attr_accessible :name
  has_many :groups, :class_name => "UserGroup", :inverse_of => :user, :foreign_key => "user_id"
end

user_group.rb

# app/models/user_group.rb
class UserGroup < ActiveRecord::Base
  attr_accessible :name, :user_id
  belongs_to :user, :counter_cache => :groups_count, :inverse_of => :groups, :foreign_key => "user_id"
end