How to associate instances from different classes in Ruby

Question

I want to be able to initialize a new Car object and pass it a Person object in the parameters, so it can be saved in that Person's @cars array. Currently, I take this approach:

person = Person.new("Michael")
car = Car.new("Honda", "Accord")
person.add_car(car)
person.add_car(Car.new("Ford", "Taurus"))
person.add_car(Car.new("Toyota", "Prius"))
person.display

However, I'd like to be able to create a new car instance and pass it the Person object I want it associated with. For example:

person = Person.new("Michael")
Car.new("Honda", "Accord", person)
Car.new("Toyota", "Camry", person)
Car.new("Chevy", "Tahoe", person)
person.display

Is that even possible?

class Person

  attr_accessor :name

  def initialize(name)
    super
    @name = name
    @cars = []
  end

  def display
    puts "#{@name} has #{@cars.length} cars"
    puts "----------------------------"
    @cars.each do |car|
      puts "#{car.make} #{car.model}"
    end
  end

  def add_car(car)
    @cars.push(car)
  end

end 

class Car

  attr_accessor :make, :model

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

  def display
    puts "#{@make} #{@model}"
  end

end

Answer 1

Yes, that is possible, Car#initialize can call methods on its arguments:

class Car
  def initialize(make, model, person = nil)
    @model = model
    @make  = make
    person.add_car(self) if(person)
  end
  #...
end

Answer 2

This would be my implementation:

class Car 
  attr_accessor :make, :model

  def initialize(make, model)
    self.make = make
    self.model= model
  end
end

Person class

class Person
  attr_accessor :name, :cars

  def initialize(name, cars=[])
    self.name = name
    self.cars = cars || []
  end

  def add_car(*args)
    raise ArgumentError, 'invalid arguments'  if (
        (args.size > 2 or args.size == 0) or 
        (args.size == 1 and !args[0].is_a?(Car))
      )
    new_car = (args.size == 2) ? Car.new(*args) : args[0]
    self.cars << new_car
    new_car
  end

end

Now you can:

person = Person.new("Michael")
car = Car.new("Honda", "Accord")
person.add_car(car)
person.add_car("Ford", "Taurus")
person.add_car("Toyota", "Prius")
person.display

The add_car method creates a new car when make and model are passed as parameters.

Answer 3

Yes, it is possible as mu is too short 's answer demonstrated, but that doesn't really make sense in my opinion. Your cars can't be used in any context without a Person , and said parameter contributes no data necessary to construct the Car object.

I would design such an API as follows:

class Person
  def initialize(name)
    @name = name
  end

  def cars
    @cars ||= Array.new  # Equivalent to @cars || @cars = []
  end
end

person = Person.new 'Michael'

taurus = Car.new 'Ford', 'Taurus'
prius = Car.new 'Toyota', 'Prius'

person.cars << taurus << prius << Car.new('Honda', 'Accord')

This is a simpler and more direct form of KandadaBoggu 's implementation that takes advantage of the Array#<< method in order to naturally associate Car s with a Person , and also doubles as an attribute reader.