poro json

poro-json.rb

require 'active_support'
require 'active_support/core_ext/object/json'

class Person
  def initialize(name, age, car)
    @name = name
    @age = age
    @car = car
    @birthyear = compute_birth_year
  end

  def compute_birth_year
    Time.now.year - @age.to_i
  end
end

class Car
  def initialize(make, model)
    @make = make
    @model = model
  end
end

p = Person.new("nick", "29", Car.new("mazda", "3"))

puts p.to_json

# {"name":"nick","age":"29","car":{"make":"mazda","model":"3"},"birthyear":1986}

Uses ActiveSupport Object#as_json:

https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/object/json.rb#L46

Which either uses the instance variables (in this example) or #to_hash if it is defined (great way for complex objects that need instance variables for internal state: just expose what you want via to_hash).