Rails 4 FlashHash Upgrade Gotcha

We upgraded an application from Rails 3 to Rails 4 this week and came across an interesting gotcha which I haven’t seen documented anywhere.

As we rolled out the Rails 4 version of the app we split the traffic between the upgraded Rails 4 app and our existing Rails 3 app. Some of the requests to Rails 3 app failed with the following exception:

Production NoMethodError: undefined method `sweep' for {"discard"=>[], "flashes"=>{"just_switched"=>true}}:Hash

Error Message: NoMethodError: undefined method `sweep' for {"discard"=>[], "flashes"=>{"just_switched"=>true}}:Hash

Where:
[GEM_ROOT]/gems/actionpack-3.2.19/lib/action_dispatch/middleware/flash.rb, line 239

It turns out that Rails 4 serialise the flash to the cookie differently to Rails 3. When Rails 3 attempts to deserialise it you get the above error. This error can also occur between Rails 3 and Sinatra apps.

To get around the problem whilst in the migration period we patched the Hash class. This would mean of course that the flash methods wouldn’t work, but as they are only used for minor presentation tweaks that seemed a good compromise.

	class Hash
	def now
	Rails.logger.warn "Stubbing now during upgrade"
	{}
	end
	def keep
	# stub keep for upgrade purposes
	Rails.logger.warn "Stubbing keep during upgrade"
	end
	def sweep
	# stub sweep for upgrade purposes
	Rails.logger.warn "Stubbing sweep during upgrade"
	end
	end

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hash.rb

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