Ruby Metaprogramming Explained: Key Aspects and Real-World Examples

Ruby Metaprogramming Explained: Key Aspects and Real-World


Ruby metaprogramming is the one of the most powerful features of Ruby that every Ruby developer is fascinated by. 

We all know how Ruby’s features like Ruby on Rails Gems, Active Controller, and Rails migration make us fall in love with it because they ease out daily Rails development hassles.

But metaprogramming takes things to a whole new level. It pushes Ruby to achieve an extreme level of expressiveness. Even the core features of Ruby, Ruby Gems, like ActiveRecord and RSpec, work as intended just because of metaprogramming.

Though its use cases in Ruby are vast, metaprogramming is little known to many Ruby on Rails developers

And some who eventually find out, get grappled with Ruby metaprogramming questions like- “What other miracles has metaprogramming performed?” What else can we do with metaprogramming?

This is where we answer your question; we explain the concept of Ruby metaprogramming, why and when to use it, its best practices, and real-world examples in this article.

What Is Metaprogramming in Ruby?

Of course, Ruby metaprogramming could mean a lot of things based on what purpose you use it for.

But generally, it’s defined as code writing because the code you write in Ruby is dynamically enhanced with some additional code at runtime by the programmed code itself.

This may sound unusual, but you can write methods and classes during runtime.

To put it simply, Ruby metaprogramming can let you reopen and repair classes, catch methods that don’t even exist in your code and create them on the fly, such as when creating codes that follow the DRY (don’t repeat yourself) principle, which avoids writing the same codebases.

Related Post: Which Ruby Interpreter and Runtime Should You Use?

The Object Model

Objects are everywhere in Ruby; these objects live alongside classes, language constructs, modules, and instance variables in an object model.

As we previously said, the codes in Ruby metaprogramming are modified at runtime, which means that whenever they are compiled, they still exist in the program where you can interact with them, ask questions about them, or manipulate them. This is known as introspection. 

Take a look at the following program where we ask the question “whether there is a response to this call method”?

“Neil Watson “.respond_to? :downcase

# => true

“Neil Watson”.respond_to? :floor

# => false

Here we can ask questions to the objects whether they can respond to specific method calls before we try using respond_to. Method

Open Classes

class String

  def log

    puts “>> #{self}”



“Hi There!!”.log   # >> Hi There!!

Here we have opened a string and added a new method log on it, so you can understand how to open any class and add any method. 

This is known as monkey patching, and it can become problematic if you redefine methods unintentionally. However, if everything goes as planned, it has the potential to significantly improve the program’s efficiency and performance. Eventually, you will need to use Ruby on Rails API if you want to add more functionality to your services.  

Related Post: Top Reasons to Migrate from Monolithic Apps to Ruby on Rails Microservices

Using Send in Ruby

You can call any method using send, it is an instance method of the object class. It accepts the method name as an argument and the remaining methods get passed to it as arguments and then passed to the method which is called.

The method can be passed as a string and gets converted into a symbol, which is preferred most of the time.

For example-

class Birds

  def eats?




dog =

dog.send(:eats?) # true

In the above Ruby metaprogramming example, the method is utilized to invoke the “eats” instance method of the Birds class.

Whenever the method is invoked, it sends the message to the object. Simply put, any Ruby method call is actually a message sent to the object.

When we use the send method, the name of the method and arguments are sent as a message to the calling object.

Send can also be utilized to invoke private and protected methods of a particular class.

Related Post: Top 10 Reasons to Choose Ruby on Rails Web Development

Class Definitions

A Ruby class definition is simply executable code. When you create a class by using the class keyword, you aren’t just deciding how future objects will behave. In fact, you are executing code.

class FavoriteClass

  puts “Hello from FavoriteClass”

  puts self


# Output

# Hello from FavoriteClass

# FavoriteClass

Dynamically Defined Classes and Methods

You can generate classes and methods on the fly while the program is running by using the Class constructor and the define_method.

Language = do

  define_method:interpret do

    puts “Interpreting the code”



# Interpreting the code

Language. new. interpret

To understand Ruby metaprogramming in an easy-to-understand style, read the Ruby metaprogramming book by Paolo Parotta.

Why Use Ruby Metaprogramming

The metaprogramming in Ruby enhances the codes to become more efficient at executing tasks compared to other languages. Moreover because of the DRY principle metaprogramming avoids repeating the same codebases to remove code redundancy.

When Ruby programmers are involved in Ruby on Rails product development they can get their project ready within no time, as metaprogramming writes the codes at the runtime automatically, allowing developers to focus on fresh new tasks.

Check out the hourly rates for hiring RoR Developers.

Code migration is another thing you can do with metaprogramming where you take the exact code bases required for some other projects and implement those in that project without having to freshly configure the Ruby file and without worrying about any errors.

Metaprogramming especially comes in handy when handling methods that are similar except for identifiers. You can utilize define_method to create methods and when identifiers are added in the later phase the methods will be present beforehand.

Ruby on Rails development with metaprogramming becomes easier if you want to write a class for accepting arbitrary methods calls. With the help of missing_methods, you can create codes that can catch all arbitrary method calls and pass for the same

Ruby Metaprogramming Real-World Examples 

Ruby metaprogramming is indeed intimidating, but most Ruby on Rails developers haven’t come across its real-world examples. This is where we enlist some of them, so take a look at it for a better understanding of metaprogramming in the real world.

Real World Examples

Sinatra Delegation 

Sinatra is a Ruby gem or code library which extends the core functionality of Ruby. it handles the tedious task of mapping web requests to blocks of the Ruby code.

It generally relies on two ways to define routes 

1. Using the get/post method in a direct manner, outside of Ruby classes

2. Defining a Ruby class that inherits from Sinatra::Application

However, the question raises here is, if it uses its DSL to define methods then how can we use it outside of the class?

Well, Sinatra resolves this problem using two concepts,

1. Metaprogramming

2. Module Extension

Sinatra defines the Sinatra::Delegator module in the base .rb file that delegates method calls to a target

And then targets Sinatra::Application

Ruby Metaprogramming Explained: Key Aspects and Real-World Examples

The above code shows the simplified version of the delegate class method defined in Sinatra::Delegator 

Here the codes generate a set of methods with define_method which eventually forwards this method to Sinatra::Application which again is the default value for Delegator. target

This is where the main objects is gets extended with methods defined in Sinatra::Delegator which makes the Sinatra DSL accessible outside of the Sinatra::Application class.

Paperclip Gem

The paperclip gem in the app is responsible for file uploads that are associated with Active Record models.

Ruby developers can define an attached file on a model with the has_attached_file method.

For example

Ruby Metaprogramming Explained: Key Aspects and Real-World Examples

It’s defined on the paperclip.rb and utilizes metaprogramming to define methods on the model.

This method accesses the file attachment which is an instance of the Paperclip::Attachment class.

For instance, here the attached file is the avatar, Paperclip defines an avatar method on the model

And define_getter_method is responsible for that

Ruby Metaprogramming Explained: Key Aspects and Real-World Examples

by analyzing the code we can easily say the attachment object is kept under the @attachment_#{name} instance variable.

On the example @attachment_avatar

Then it verifies whether the attachment exists, and if not it creates a new Attachment and set a new variable.

Related Post: Best Ruby Frameworks for Web Development and Business Growth in 2023

When To Choose Ruby Metaprogramming

Ruby metaprogramming has its advantages and disadvantages, so it’s important to know when to use it and automate your Ruby on Rails development.

For instance, when adding new methods using monkey patching, it can become a problematic situation if you unintentionally define a method. This can lead to code complexity, and the developers might not be able to comprehend the code and its function.

This can also happen when you are implementing Ruby metaprogramming methods and attributes, so it’s wise to choose metaprogramming when you modify programs heavily to reduce code redundancy for complex projects and form an automated process for enhanced operation of Ruby apps.

Watch out our video on how to hire Ruby on Rails Developers.


Ruby is among the best languages for web application development. And it comes with some cutting-edge features like Ruby Gems, Rails migrations, and an Action Controller that make the Rails applications more robust.

The core of the Ruby gem and other functionalities lies in Ruby metaprogramming, and for this reason, it becomes crucial for making Ruby applications more advanced to provide users with better accessibility.

The above Ruby metaprogramming information is useful for companies and even for developers who want to check out how it improves Ruby on Rails development by leveraging the full potential of the Ruby programming language.



Subscribe for newsletter

Trusted By

Our platform focuses on making the remote hiring process easier by providing top quality vetted developers from around the world. Through our service many well-known companies have scaled their product development team.

Related Article

Ready to scale your team with remote engineers?