Weird instance variable behaviour in Rails model

Question

Let's consider the following example:

class Car < ActiveRecord::Base

  def engine
    @engine ||= Engine.new(self)
  end

end

class Engine

  def initialize(vehicle)
    @vehicle = vehicle
  end

  def model
    @vehicle.model
  end

end

and what I don't understand is that when I go to the console and do something like that:

car = Car.last
car.engine.model  # => 'V8'

and then

car.model = 'V6'

I get

car.engine.model  # => 'V6'

It's obviously a simplified example but anyway, can anyone explain why I get a different engine model name in the last line of code? Isn't it supposed to use the @engine variable and return the previous engine model name?

Marek Lipka · Accepted Answer · 2015-02-13 08:37:50Z

1

No surprise here. You initialize Engine instance with car object, so @vehicle instance variable have reference to the same object as car local variable. No wonder then that if you modify car object, @engine object will be modified in the same way.

answered Feb 13, 2015 at 8:37

Marek Lipka

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2 Comments

NoDisplayName Over a year ago

I see, thank you. Is engine method safe the way I implemented it or it's better to create a new instance of engine with every call just to make sure that I always get the correct information in case some of the car attributes change?

Marek Lipka Over a year ago

@JustMichael it's no difference at this aspect. If you instantiate Engine every time you call Car#engine method, it would still return changed Car attributes.

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