ruby-on-rails - ActiveRecord::ConnectionNotEstablished on rails3 upgrade

Question

Rails 2.3 から Rails 3 にアップグレードしようとしていrake rails:upgrade:checkます。

irb(main):010:0> Game.all
ActiveRecord::ConnectionNotEstablished: ActiveRecord::ConnectionNotEstablished
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/connection_adapters/abstract/connection_pool.rb:317:in `retrieve_connection'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/connection_adapters/abstract/connection_specification.rb:97:in `retrieve_connection'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/connection_adapters/abstract/connection_specification.rb:89:in `connection'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/base.rb:602:in `quoted_table_name'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/relation/query_methods.rb:275:in `build_select'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/relation/query_methods.rb:189:in `build_arel'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/relation/query_methods.rb:149:in `arel'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/relation.rb:64:in `to_a'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/relation/finder_methods.rb:143:in `all'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.0.20/lib/active_record/base.rb:444:in `all'
    from (irb):10
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.0.20/lib/rails/commands/console.rb:44:in `start'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.0.20/lib/rails/commands/console.rb:8:in `start'
    from /Users/ttt/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.0.20/lib/rails/commands.rb:23:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

私は mysql を使用しており、から mysql2 gem にアップグレードしています。gem ファイルは次のようになっています。

gem "rails", "3.0.20"
gem 'mysql2', '~> 0.2.17'

私も更新database.ymlしました

development_default: &development_default
  adapter: mysql2
  encoding: utf8
  reconnect: false
  pool: 5
  socket: /tmp/mysql.sock

development:
  <<: *development_default
  database: some_database
  username: some_user
  password: some_password
  host: some.domain.com
  port: 4000

Rails 2ブランチに戻ってコードを正常に実行できるので、間違いなくアップグレードコードに問題があります。問題が何であるか、または次にどこを見るべきかについての指針はありますか?