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Asked: 2020-09-08 02:16:43 +0800 CST 2020-09-08 02:16:43 +0800 CST

In Ruby what is the difference between using def and define_method?

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I understand that you define_methodcreate a method dynamically, but... in what cases would you use it?

What is the 'advantage' or what is different in use, over creating a simple method with def?

Please don't cite memory references or pass links to the official documentation, because that doesn't exactly say what I'm asking.

ruby

1 Answers

  1. Best Answer

    The advantage is that you can use variables in creating a method and create less code when there are multiple methods with minimal variations.
    As a sample, an example: Imagine having a model Userwith an attribute statusthat defines if that user is in active or inactive state. You also need a couple of methods to help you:

    • Check if a user is in a specific state.
    • Check the database for all users who are in a specific state.

    Then the model that I describe would look something like this:

    class User < ApplicationRecord

  # Con esto podría hacer user.active? y retornaría true/false dependiendo 
  # si el usuario está activo o no
  def active?
    status == 'active'
  end

  def inactive?
    status == 'inactive'
  end

  # Acá al ejecutar User.active retornaría todos los usuarios activos
  def self.active
    where(status: 'active')
  end

  def self.inactive
    where(status: 'inactive')
  end

end

    All good so far. Now it turns out that there are new requirements and you need to add 5 new states to your user model, for which the methods that I explained should also be defined. You could then create the methods, but it would be somewhat repetitive, where the only thing that would change is the name of the state. In this case it may be necessary to do some metaprogramming and create the methods as follows:

    class User < ApplicationRecord

  # Creo un arreglo con los nombres de todos los estados posibles, incluidos 
  # los 5 nuevos
  STATUS = ['active', 'inactive', 'locked', 'deceased', 'admin', 'whatever', 'asdf']

  # Y luego iterar ese arreglo para crear los métodos necesarios de cada estado
  STATUS.each do |st|

    # define_singleton_method sirve para definir métodos de clase
    define_singleton_method st do
      where(status: st)
    end

    # define_method sirve para definir métodos de instancia
    define_method "#{st}?" do
      status == st
    end

  end

end

    So in this way you have saved yourself from writing 14 methods that practically did the same thing, plus you have a much cleaner code.

    Another clear example is what Rails does internally with the variable names you define. For example to create the route helpers as users_pathor new_user_urlwhen you define resources :usersin the routes file.

    Do not worry if at first you feel that this is not very useful or you do not find where to apply it. Its use is something that you may not be able to visualize at first, but as your code grows, you may feel the need to refactor your code and take up some metaprogramming, where it can be useful.

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