gistfile1.rb

class UpdateCache < ActiveRecord::Migration
  def up
    User.all.each do |user|
      object = user
      association = :pets
      counter_name = :pets_count
      User.connection.update("UPDATE #{User.quoted_table_name} SET #{User.connection.quote_column_name(counter_name)} = #{object.pets.count} WHERE #{User.connection.quote_column_name(User.primary_key)} = #{User.quote_value(object.id)}", "#{User.name} UPDATE")
    end
  end

  def down
  end
end

gistfile2.txt

    servers: [""]
    [] executing command
 ** [out :: ] ==  UpdateCache: migrating ====================================================
 ** [out :: ]
 ** [out :: ] rake aborted!
 ** [out :: ]
 ** [out :: ] An error has occurred, this and all later migrations canceled:
 ** [out :: ]
 ** [out :: ]
 ** [out :: ] uninitialized constant User::Pet
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/inheritance.rb:111:in `compute_type'

 ** [out :: ]
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/reflection.rb:172:in `klass'
 ** [out :: ]
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/associations/association.rb:117:in `
klass'
 ** [out :: ]
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/associations/association.rb:123:in `
target_scope'
 ** [out :: ]
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/associations/association.rb:87:in `s
coped'
 ** [out :: ]
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/associations/collection_association.
rb:210:in `count'
 ** [out :: ]
 ** [out :: ] /var/www/Test/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.13/lib/active_record/associations/collection_proxy.rb:46:

gistfile3.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :token_authenticatable, :confirmable,
  # :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable,
         :confirmable
  validates :email, :presence => true, :uniqueness => true
  validates :username, :presence => true, :uniqueness => true

  # Pets
  has_many :pets, :foreign_key => :owner_id
  # @return [Pet] Active pet for this user.
  def active_pet
    self.pets.length > 0 ? self.pets[0] : nil
  end
end

gistfile4.rb

class Pet < ActiveRecord::Base
  belongs_to :owner, :class_name => "User", :counter_cache => true
  validates :owner, :presence => true

end