Monkey patching in Rails

Monkey Patching. It’s amazing. One of the most powerful Ruby features. You have ability to open any ruby class and change how it works, add new methods, basically you can do almost anything. But, as we all now from the great power comes great responsibility, and putting the power in the human hands is usually not so great idea.

Let’s see how to use Monkey Patching in Rails, where to store the code, and how to make you life easier in 1 year from now when things stop to work because of the patch you/someone wrote.

The problem and example app

Surprisingly, 90% of Rails application I was involved in required ability to check if String value is a Number. Let’s say we have analyses table which stores name, and the value of analysis done on the blood sample.

Some of the values can be Number(ex: 1.5, 2.45, etc..) some of them just plain String(positive, negative, etc...). But we will go easiest route in this example and store each result in value column which is String.

Let’s roll!

rails new sampler # create new rails application
bin/rails g model analysis name value # create analysis model
bin/rake db:migrate # create the table

Now, insert some test data into the analyses table:

bin/rails console
Analysis.create(name: "Leukocite", value: "7.56")
Analysis.create(name: "Blood color", value: "blue")  # watch it vampire!!!

Since in this fictive application I need ability to figure out if the value of Analysis is string or number, I need a way to check if String is valid Number. I will open String class and add method is_number? Which will check if string is number (NOTE: not the only and certainly not the fastest way to resolve the problem but for the sake of this post, I will do it).

class String
  def is_number?
    true if Float(self) rescue false
  end
end

Now I can call is_number? On each string value in my app,like this

Analysis.first.value.is_number?  ⇒ true
Analysis.last.value.is_number?   ⇒ false
"ok".is_number?                  ⇒ false

Well, not so fast! How to add this in Rails app and where to add it ?

Put them in a module

When you monkey patch a class, don’t just reopen the class and shove your patch into it:

class String
  def is_number?
    true if Float(string) rescue false
  end
end

Why not?

• If two libraries monkey-patch the same method, you won’t be able to tell.
• If there’s an error, it’ll look like the error happened inside String(While technically true, it’s not that helpful).
• It’s harder to turn off your monkey patches.
• If you, say, forgot to require ‘string’ before running this monkey patch, you’ll accidentally redefine String instead of patching it.

Instead, put monkey patches in a module:

module CoreExtensions
  module String
    module Number
      def is_number?
        true if Float(self) rescue false
      end
    end
  end
end

This way, you can organize related monkey patches together. When there’s an error, it’s clear exactly where the problem code came from. And you can include them one group at a time:

# Actually monkey-patch String, you can add this to Rails initializers
String.include CoreExtensions::String::Number

Keep them togeather & ogranized

When you monkey patch core Ruby classes you are add/change Ruby API. You need a way to quickly find and learn those changes when you jump in codebase.

I mostly follow Rails’ monkey patching convention. Patches go into lib/core_extensions/class_name/group.rb or lib/core_ext/class_name/group.rb.

So this patch:

module CoreExtensions
  module String
    module Number
      def is_number?
        true if Float(self) rescue false
      end
    end
  end
end

will go into: lib/core_extensions/string/number.rb

Now you/new developers can easily jump in lib/core_extensions and see all of the monkey patching adventures :)

Conclusion

Always think through when you want to do monkey patching, is there better way. In this example definitely there is, we could store results differently not everything in one string column. But this is just the naive example :)

If you go for the big guns it is nice to know how to use them.

Dino Maric

Working class hero

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