Rails and Ruby Deprecation Warnings: Why You Should Care and How to Fix Them

If you’ve ever fired up your rails server (rails s) or console (rails c) and been greeted with a loud "DEPRECATION WARNING,” you’re definitely not alone.

These warnings pop up because rails, in its ongoing evolution , regularly retires older features—features that might still work today but could spell trouble down the road.

In this post, I want to go through what deprecation warnings mean and how to handle them effectively in order to avoid common pitfalls.

TL;DR

• Deprecation warnings are rails’(and ruby's) way of giving you a heads-up about features that will soon be removed or changed.
• Ignoring them can lead to unexpected issues during upgrades.
• Use configurations like config.active_support.deprecation to handle these warnings in development or test environments.
• You should at least pay attention to deprecation warnings from both rails and ruby.

Why Should You Care About Deprecation Warnings?

Deprecation warnings are essentially rails’ polite heads-up that a feature or method you’re using is on its way out.

While technically it is just a “warning” that won’t break your app today, ignoring it could set you up for headaches down the road. After all, if it’s called a “warning,” you should probably pay attention to it, right? 😌 When the deprecated feature is eventually removed in a future rails version, your app could break unexpectedly.

When Deprecation Warnings Actually Matter?

From my experience, deprecation warnings matters most when you’re updating your app—especially during rails version upgrades.

Imagine this: you’re in charge of upgrading rails, juggling multiple breaking changes, and managing dependencies across a bunch of gems. The last thing you want is to be blindsided by deprecated features that have been quietly lurking in your app, only to find out they’re no longer supported in the new version. Not fun.

The earlier you tackle these warnings, the smoother your upgrade process will be, and the fewer bugs you’ll run into later. (I get it—keeping an eye on warnings and addressing them right away isn’t always realistic. But trust me, a little proactive effort here can save you down the road.)

Where Do We Find Deprecation Warnings?

In rails, you might see deprecation warnings in various places depending on your environment setup and the context of the issue. Here's where you commonly encounter them:

1. Development Environment Console

Deprecation warnings often appear in the rails server console (rails s) when you run your application in the development environment. These warnings are printed in real-time as you use the app.

2. Test Suite Output

When running your test suite, deprecation warnings might appear in the terminal output if your tests trigger deprecated methods or features. The configuration file for the test environment (test.rb) usually has a different value set for config.active_support.deprecation than development.rb, which is why you may only see the warning when you run your tests. We’ll talk about this in more detail in a later section.

3. Rails Logs

Deprecation warnings are logged in your application's log files (e.g., log/development.log, log/test.log, or log/production.log depending on the environment). These logs can be reviewed to identify and address issues. (This is also based on your value of config.active_support.deprecation )

4. Rails Console

If you're experimenting or debugging in the rails console (rails c), deprecation warnings can appear when you use deprecated methods.

5. CI/CD Pipelines

If you have continuous integration or deployment pipelines, deprecation warnings might appear in the build logs when running tests or tasks.

How do we mange deprecation warning?

There are two primary types of deprecation warnings you should definitely watch for to ensure a smoother upgrade experience in the future:

・Rails-specific warnings
・Ruby-specific warnings

1. Deprecation Warnings in Rails

Rails provides built-in tools to help you manage deprecation warnings effectively. You can configure how your application responds when a deprecated feature is used by adjusting the config.active_support.deprecation setting in your configuration files.

Rails offers several actions you can take to control how deprecation warnings are handled:

• raise: Immediately raises an error, halting execution. This is useful for catching deprecated features early in development or testing.
• stderr: Prints the warning to the standard error output.
• log: Sends the warning to your application's log file. Ideal for apps that have been around for a while and need time to address warnings gradually.
• notify: Triggers a notification, like a Slack message or email, whenever a deprecation occurs.
• silence: Suppresses the warning entirely. This should be used sparingly.

If you are creating a new app, I would probably configure this setting to :raise in development and test environments. This means that when a deprecated feature is encountered, rails will raise an error immediately. By catching deprecation warnings early in the development cycle, you’ll be able to address them before they become bigger issues.

You can add like this to your config/environments/development.rb or config/environments/test.rb:

config.active_support.deprecation = :raise

HOWEVER, if the app has been around for a while, it might have a lot of warnings so :raise is not an ideal option. You could send the warning to logs and later review it or notify to slack to regularly review.

Furthermore, if you’re not comfortable with globally raising errors for all deprecation warnings, rails allows more granular control. The config.active_support.disallowed_deprecation setting lets you specify individual deprecations that should be handled differently.

This is useful if you've already addressed certain deprecations but want to be alerted if they reappear. You can configure specific deprecations to raise an error, which helps you identify and fix regressions quickly.

config.active_support.disallowed_deprecation = :raise
config.active_support.disallowed_deprecation_warnings = [
  /will be removed in Rails \d+\.\d+/ 
]

This way, you'll be notified if certain deprecated features are reintroduced into your app.

Managing Deprecation Warnings in Ruby

Dealing with deprecations in rails is only part of the story. You also need to pay attention to Ruby deprecations, as these can have a significant impact on your app's future compatibility as well.

A strategy I recommend is to apply the same approach to Ruby deprecation warnings as you do for rails. By handling both sets of warnings in a similar manner, you ensure a more consistent and proactive approach to keeping your app up-to-date.

Thanks to Piotr Jurewicz, who clearly explains how to print Ruby deprecation warnings (e.g., “DEPRECATION WARNING: [RUBY]”), the process is straightforward.

In essence, the Warning module in Ruby includes the warn method, which issues all warnings, typically printed to $stderr. You can customize the behavior of Warning.warn by extending the module with your own method, allowing you to filter warnings or redirect their output. To maintain the default behavior, simply call super within your custom method. However, it's advised not to redefine the instance method Warning#warn, as doing so may interfere with the default warning functionality.

(Read more about the Warning module here: Ruby Documentation)

For example, here’s how you can capture Ruby 2.7 deprecation warnings:

You could also enhance this logic to send notifications to Slack or any other service when a deprecation warning occurs.

module MyWarningFilter
  RUBY_DEPRECATIONS_2_7_2 = [
    "Using the last argument as keyword parameters is deprecated",
    # Add other deprecations as needed
  ]

  def warn(message)
    if RUBY_DEPRECATIONS_2_7_2.any? { |warning| message.include?(warning) }
      ActiveSupport::Deprecation.warn(message)
    else
      super
    end
  end
end

Warning.extend(MyWarningFilter)

I highly recommend reading Piotr’s blog post, as it provides detailed guidance on creating initializer files like the one above for various Ruby and Rails combinations. You’ll gain deeper insights into how to handle deprecation warnings effectively:

Do You Tune Out Ruby Deprecation Warnings?

Wrapping up

When you see deprecation warnings, it’s better not to ignore them—taking responsibility and action now will save you time and frustration later.

Proactively managing these warnings ensures your app stays compatible with future versions of rails and Ruby. Addressing them early helps prevent issues during upgrades and maintains the health of your application. By utilizing rails’ built-in tools and customizing Ruby’s Warning module, you can stay ahead of potential problems. Taking action now ensures a smoother, more stable development process in the long run.