Iteration 2: Objects and Operations That Spam Requests

Flash: communicating between actions

When we use redirect_to() to transfer control to another action, the browser generates a separate request to invoke that action. That request will be handled by our application in a fresh instance of a controller object. Instance variables that were set in the original action are not available to the code handling the redirected action. Sometimes we need to communicate between these two instances, though. We can do this using a facility called the flash.

The flash is a temporary scratchpad for values. It is organized like a hash and stored in the session data, so we can store values associated with keys and later retrieve them. It also has one special property. By default, values stored into the flash during the processing of a request will be available during the processing of the immediately following request. Once that second request has been processed, those values are removed from the flash.

Probably the most common use of the flash is to pass error and informational strings from one action to the next. The intent here is that the first action notices some condition, creates a message describing that condition, and redirects to a separate action. By storing the message in the flash, the second action is able to access the message text and use it in a view.

It is sometimes convenient to use the flash as a way of passing messages into a template in the current action. For example, our display() method might want to output a cheery banner if there isn’t another, more pressing note. It doesn’t need that message to be passed to the next action, because it’s for use in the ...