Whitelisting with ActiveSupport module method presence_in

Let's say your app presents a list of articles and offers multiple ways of sorting them: by upload date, by the number of upvotes and number of comments. One of the possible techniques to support this behavior is defining three types of scopes on your Article model:

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class Article < ActiveRecord::Base
  scope :newest,         -> { order(updated_at: :desc) }
  scope :top,            -> { order(upvotes_count: :desc) }
  scope :most_commented, -> { order(comments_count: :desc) }
end

You'll want to call those scopes depending on the query parameter passed to your controller, e.g. /articles?sort_by=newest. The easiest way to implement this in our controller would be:

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class ArticlesController < ApplicationController
  def index
    @articles = Article.public_send(params[:sort_by])
  end
end

However, you don't want to send those user inputs directly to your model because you would leave yourself vulnerable to remote code execution. An evildoer could send you a request such as /articles?sort_by=destroy_all and destroy your records. As a responsible developer, you sanitize that input:

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class ArticlesController < ApplicationController
  def index
    @articles = Article.public_send(sorting_technique)
  end

  private

  def sorting_technique
    params[:sort_by].presence_in(%w(newest most_commented top)) || :newest
  end
end

DESCRIPTION:

Returns the receiver if it's included in the argument. Otherwise, it returns nil. The argument has to be any object which responds to #include?.

SOURCE:

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def presence_in(another_object)
  self.in?(another_object) ? self : nil
end