Method chaining in Ruby

08 Jun 2022
#ruby #rails
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One of Ruby's great features is method chaining. For instance, Active Record heavily relies on this and method chaining enables us to do cool things like User.where("active = 1").count or 1.day.ago.

Have you ever wondered about how is that possible? Let's look into it, it is pretty simple.

Let's use a simple example of having a user model and you want to be able to set a default password for your user when they signup via OAuth. So you want to be able to do something like this in your controller: 

class OmniauthCallbacksController < ApplicationController
  def provider
    auth = request.env['omniauth.auth']
    user = User.find_or_create(provider: auth.provider, email: auth.email, uid: auth.uid)
    user.set_password.save! unless user.persisted?
    ...
  end
end
Then in your model you would implement the set_password method: 
class User < ApplicationRecord
  def set_password
    self.password = SecureRandom.alphanumeric(16)
  end
end
If you you try User.new.set_password.save! it will fail with a NoMethodError, because set_password returns a string, but the save method is implemented on User.
I bet now you figured out how to make this work. It is pretty simple, you just need to return the user instance from set_password: 
class User < ApplicationRecord
  def set_password
    self.password = SecureRandom.alphanumeric(16)
    self
  end
end
And voila, now you can chain any method after set_password.
With the simple trick of returning the instance of the object from a method, you are making it chainable.

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