Rails controllers and instance variables

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?

Dave Newton · Accepted Answer · 2019-04-01 21:18:11Z

7

Correct; it belongs to the instance of FoosController that's processing the current request.
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.
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.

edited Apr 1, 2019 at 21:18

answered Jun 7, 2012 at 0:20

Dave Newton

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3 Comments

Anil Over a year ago

Good explanation. To add, @foos is an array containing instances of the Foo objects.

Dave Newton Over a year ago

@Anil Thanks. I think the OP mentioned what @foos contains, so I didn't add it :)

Anil Over a year ago

I know you don't miss anything. But I like it when I can get all my knowledge in one spot. This is the type of question which helps the OP a little now, but has a long term value to the communtiy.

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