From: Xavier Noria <fxn@...>
Date: 2006-11-12T05:55:49+09:00
Subject: class reloading and constants

Context: I am trying to understand how does Rails implement class  
reloading.

If we reload a .rb file that defines class A::B with Kernel#load we  
are just reopening the class, so that's poor man's reloading in the  
sense that redefined methods are updated, but removed methods are  
still there.

I guess (it is just a guess) that's the reason Rails does black magic  
with constants in Class#remove_class. I think the key line there is

   parent.send :remove_const, basename unless parent == klass

which removes the basename of the class we want to remove from the  
constants in its parent module, if any. The irb session below  
suggests that technique triggers a "fresh" reload of the class, but  
why? A posteriori one would bet a class definition reopens the class  
if there's already a class object in that constant, and just  
instantiates a new one otherwise, is that right?

-- fxn

fxn@feynman:~/tmp$ irb
irb(main):001:0> IO.read('foo.rb')
=> "module A\n  class B\n    def x\n      'x'\n    end\n  end\nend"
irb(main):002:0> load 'foo.rb'
=> true
irb(main):003:0> A::B.new.x
=> "x"
irb(main):004:0> A::B.object_id
=> 1683700
irb(main):005:0> A.send(:remove_const, 'B')
=> A::B
irb(main):006:0> IO.read('foo.rb')
=> "module A\n  class B\n    def y\n      'y'\n    end\n  end\nend"
irb(main):007:0> load 'foo.rb'
=> true
irb(main):008:0> A::B.new.x
NoMethodError: undefined method `x' for #<A::B:0x59f27c>
         from (irb):8
         from :0
irb(main):009:0> A::B.new.y
=> "y"
irb(main):010:0> A::B.object_id
=> 2952610