Whitelisting with the lesser-known #presence_in method

Damir Svrtan
  —  2 min read

Quite often when writing Rails applications we need to do some kind of user input sanitization. We usually end up doing some dull checks across multiple lines. Have a look at this hidden ActiveSupport #presence_in method that provides a convenient way to do whitelisting in just one line.

Whitelisting opening graphic

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:

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:

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:

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

  private

  def sorting_technique
     if %w(newest most_commented top).include? params[:sort_by]
       params[:sort_by]
     else
       :newest
     end
  end
end

Although this code provides safety, it's still not very nice. I've written similar code many times and have never felt happy about it because I believed it could be shorter.

The #presence_in method to the rescue

That's where the lesser-known method from the ActiveSupport module called Object#presence_in comes in.

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:

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

Usage:

With the presence_in method we can write our sorting_technique method with a nice one-liner:

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

Its usage doesn't stop here, but I've found it to be most useful when sanitizing user input.

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