Preserve

Preserve is a Rails plugin which stores selected parameters in a session to make them available in subsequent requests.

Installation

Add the following line to your application's Gemfile:

gem 'preserve'

Install the gem with Bundler:

$ bundle install

Or do it manually by running:

$ gem install preserve

Usage

For the sole purpose of this example, assume we have a Rails application with a controller showing all parameters sent in a request.

class ParametersController < ApplicationController
  def index
    render json: request_parameters
  end

  private

  def request_parameters
    params.except(:controller, :action)
  end
end

Routes are declared as following:

Rails.application.routes.draw do
  resources :parameters, only: :index
end

Let's start the application and test its behaviour using cURL. The whole concept is based on a session, so in order for this to work, the cookie engine must be enabled (hence -c and -b options).

In the first request, the per_page parameter is set to 20.

$ curl -c cookies http://localhost:3000/parameters?per_page=20

{"per_page":"20"}

As expected, the application returns the parameter and its value.

The next request is sent without any parameters.

$ curl -b cookies http://localhost:3000/parameters

{}

Obviously, the per_page parameter is no longer available.

Now, let's call the preserve method inside the ParametersController class with the parameter name as an argument.

class ParametersController < ApplicationController
  preserve :per_page

  # ...
end

Sending the same two requests again gives a different result.

$ curl -c cookies http://localhost:3000/parameters?per_page=20

{"per_page":"20"}

$ curl -b cookies http://localhost:3000/parameters

{"per_page":"20"}

This time, the per_page parameter is still available when the second request is made, even though it wasn't sent particularly in that request.

Multiple arguments

If more than one parameter needs to be preserved within the same controller, its name can be passed as a succeeding argument to the preserve method.

preserve :per_page, :page

Restrictions

Limiting functionality provided by the gem to a certain set of controller actions can be achieved by applying the :only (or :except) option.

preserve :per_page, only: :index

It behaves exactly like the :only option of an Action Controller filter. In fact, the gem sets such filter underneath, so you can make use of all its options – they will be passed to that filter.

Application-wide parameters

When there's a need to store a parameter used across the whole application, the preserve method should be called inside the ApplicationController.

class ApplicationController < ActionController::Base
  preserve :locale
end

In more complex scenarios, controller inheritance can be utilized to further adjust the scope.

Setting a session key prefix

By default, parameter values are stored in a session with a key that consists of a controller name and parameter name (e.g. users_order for the order parameter in the UsersController).

In most cases such combination results in a unique session key, but there might be a situation when it's necessary to add a prefix in order to avoid conflicts with a session key that is already in use. It can be done by passing the :prefix option.

class UsersController < ApplicationController
  preserve :order, prefix: 'preserved'
end

From now on, the parameter will be stored in a session with the preserved_users_order key.