Rails and class variables

Question

class MainController < ApplicationController

  @my_var = 123
   def index
    var1 = @my_var
   end

   def index2
    var2 = @my_var
   end
end

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Answer 1

Because code executes in different context. You can see here:

class MainController
  puts self
  def print_self
    puts self
  end
end
#=> MainController
MainController.new.print_self #=> 

As you can see in first print the self is MainController, in second print the self is the object which derived from MainController class.

In the assignment to @my_vay this variable belongs to MainController, and in the second cases, the @my_var belongs to object (not a class) and these varaibles are different.