How does metaprogramming and method_missing work in Ruby?

Ruby programs can define methods and classes at runtime. Common tools include define_method, send, respond_to?, instance_variable_get, class_eval and hooks such as included, inherited and method_added.

method_missing intercepts calls to undefined methods:

class Recorder
  def method_missing(name, *args)
    if name.to_s.start_with?("record_")
      key = name.to_s.sub("record_", "")
      puts "recording #{key}: #{args}"
    else
      super
    end
  end

  def respond_to_missing?(name, include_private = false)
    name.to_s.start_with?("record_") || super
  end
end

Always call super for names you do not handle, and define respond_to_missing? so respond_to? stays accurate. Metaprogramming enables expressive DSLs such as Rails routes and RSpec, but it hurts readability, tooling and performance. Prefer define_method for known methods.