Make Rails 3.1 use symbols in sessions so it can share Rails 2 sessions.

application.rb

#...

module MyApp
  class Application < Rails::Application
 
    #...
    
    config.after_initialize do
      require 'session_patch'
    end
    
  end
end

session_patch.rb

# This patch makes Rails 3 use symbols as storage keys in sessions,
# as Rails 2 did.

module Rack
  module Session
    module Abstract
      class SessionHash
        def [](key)
          load_for_read!
          super(key.to_sym)
        end
        def has_key?(key)
          load_for_read!
          super(key.to_sym)
        end
        def []=(key, value)
          load_for_write!
          super(key.to_sym, value)
        end
        def delete(key)
          load_for_write!
          super(key.to_sym)
        end
        private
        def stringify_keys(other)
          hash = {}
          other.each do |key, value|
            hash[key.to_sym] = value
          end
          hash
        end
        
      end
    end
  end
end

Put session_patch.rb in lib/ or similar

this wont require any more, still keeping it can throw error of cannot modify flash, specially in case of activeadmin