Problem
Upgrading an existing Ruby on Rails application from 1.2.3, to 2.0.2, presents few problems. I will try and keep a record of the ones I encounter along the way, here.

Solution

1. Change the config/environment.rb to let the application know to use the 2.0.2 gem rail version,

change the following line from:

RAILS_GEM_VERSION = '1.2.3' unless defined? RAILS_GEM_VERSION

to:

RAILS_GEM_VERSION = "2.0.2" unless defined? RAILS_GEM_VERSION

2. Run the following to generate the secret key for the application:

rake secret

3. Copy the magic key in a new section in your config/environment.rb as in:

...

    # config.log_level = :debug

    # Your secret key for verifying cookie session data integrity.
    # If you change this key, all old sessions will become invalid!
    # Make sure the secret is at least 30 characters and all random,
    # no regular words or you'll be exposed to dictionary attacks.
    config.action_controller.session = {
      :session_key => '_yourapplication_session',
      :secret      => 'long_string_generated_from_rake_secret'
    }