Dynamically add methods in ruby with define_method

In a recent post I went over adding methods dynamically with method_missing. I also issued a word of caution when overriding method_missing in that manor.

It turns out there is a better way to handle an event where you need to define methods at runtime in ruby, through the use of define_method. The benefit to using define_method in this case is it will have any unintended side effects like method_missing does with impacting respond_to?.

In the code below I use define_method to add attribute methods to the Account class. When this file is first read by the ruby interpreter it will iterate the @@attributes array and create the setter methods. Then we can use them later on in our code, like where the example below illustrates when we set a value for email.

class Account
  @@attributes = [:email, :first_name, :last_name]

  @@attributes.each do |name|
    define_method(:"#{name}=") do |value|
      @attributes[name] = value

  def initialize
    @attributes = {}

Account.new.email = "wizbang@buck.com"

If it’s not clear, what makes this dynamic is our code is defining the attribute methods for us inside of define_method as it runs rather than us declaring them within our code.

I’ve seen this used in production where a remote API contains data you want to attach to a class in a different ruby/rails app. The data is fetched from the API and define_method is used to attach that data to the model.