Question

A typical rails controller might do something like this:

class FoosController < ApplicationController
  def index
    @foos = Foo.all
  end
end

I understand rails well enough to know that @foos will return an array of Foo objects, but that @foos itself is an instance variable.

So which object does the instance variable belong to? Would it be an instance of the FoosController class? Is a different instance of this object created every time I access the index page? What about if I access the show page, where a new variable @foo is introduced:

def show
  @foo = Foo.find(params[:id])
end

Does this variable belong to the same object that @foos belongs to?

Answer 1

1. Correct; it belongs to the instance of FoosController that’s processing the current request.
2. Yes, each request creates a new instance of the controller–that’s why instance variables can be used to hold state for a single request.
3. Yes, but: if index isn’t called, @foos won’t be initialized, There is no @foos instance variable when the show action is hit in the code you show¹.

---

¹ If you called index from show, then @foos would be initialized and available on the page. Not that you should do this, because that’s confusing concerns.