FlashHash override in Rails 3.0.20 code to support flashes created in Rails 3.2.22 (https://raj454raj.wordpress.com/2016/08/13/flashhash-issue-rails-upgrade/)

flashhash_override_3_0.rb

# This patch handles both types of flashes that are created in
# Rails 3.2.22 as well as those created in Rails 3.0.20

ActiveRecord::SessionStore::Session.class_eval do

  def self.unmarshal(data)
    return unless data

    marshalled = nil
    begin
      decoded = ::Base64.decode64 data
      marshalled = Marshal.load decoded
    rescue ArgumentError => ae
      if ae.message =~ /dump format error/
        with_alternate_flash_klass do
          marshalled = Marshal.load decoded
        end

        if marshalled['flash']
          new_flash = ActionDispatch::Flash::FlashHash.new

          flash_message = marshalled['flash'].instance_variable_get('@flashes')
          flash_message.each { |k, v| new_flash[k] = v } if flash_message

          used = marshalled['flash'].instance_variable_get('@used')
          new_flash.instance_variable_set('@used', used) if used

          # FlashHash object created in Rails 3.2 unparsed to support Rails 3.0 behavior
          marshalled['flash'] = new_flash
        end
      else
        raise ae
      end
    end

    marshalled
  end

  def self.alternate_flash_klass
    if ActionDispatch::Flash::FlashHash.ancestors.include? Hash
      Class.new() do # Rails 3.2 FlashHash
        def initialize
          @used    = Set.new
          @closed  = false
          @flashes = {}
          @now     = nil
        end
      end
    else
      Class.new(Hash) do # Rails 3.0 FlashHash
        def initialize
          super
          @used = Set.new
        end
      end
    end
  end

  def self.with_alternate_flash_klass
    temporary_flash_hash_klass = alternate_flash_klass

    original_flash_hash_klass = ActionDispatch::Flash::FlashHash
    ActionDispatch::Flash.send :remove_const, :FlashHash
    ActionDispatch::Flash.const_set 'FlashHash', temporary_flash_hash_klass

    yield

    ActionDispatch::Flash.send :remove_const, :FlashHash
    ActionDispatch::Flash.const_set 'FlashHash', original_flash_hash_klass
  end
end