When working with gems in Rails, what does 'cannot remove Object::ClassMethods' stem from?

Question

Frequently I have run into a problem when installing gems that provides a problem like:

Does anyone know what this stems from? I've seen in it several different cases, yet still haven't learned what exactly is causing it.

$ sudo rake gems:install --trace
(in /u/app/releases/20100213003957)
** Invoke gems:install (first_time)
** Invoke gems:base (first_time)
** Execute gems:base
** Invoke environment (first_time)
** Execute environment
rake aborted!
cannot remove Object::ClassMethods
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:603:in `remove_const'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:603:in `remove_constant'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:603:in `instance_eval'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:603:in `remove_constant'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:549:in `new_constants_in'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:549:in `each'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:549:in `new_constants_in'
/u/app/releases/20100213003957/vendor/rails/activesupport/lib/active_support/dependencies.rb:156:in `require'
/u/app/releases/20100213003957/vendor/rails/railties/lib/tasks/misc.rake:4
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:617:in `call'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:617:in `execute'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:612:in `each'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:612:in `execute'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:578:in `invoke_with_call_chain'
/usr/lib64/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:571:in `invoke_with_call_chain'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:564:in `invoke'
/u/app/releases/20100213003957/vendor/rails/railties/lib/tasks/gems.rake:17
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:617:in `call'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:617:in `execute'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:612:in `each'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:612:in `execute'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:578:in `invoke_with_call_chain'
/usr/lib64/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:571:in `invoke_with_call_chain'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:588:in `invoke_prerequisites'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:585:in `each'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:585:in `invoke_prerequisites'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:577:in `invoke_with_call_chain'
/usr/lib64/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:571:in `invoke_with_call_chain'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:564:in `invoke'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:2027:in `invoke_task'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:2005:in `top_level'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:2005:in `each'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:2005:in `top_level'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:2044:in `standard_exception_handling'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:1999:in `top_level'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:1977:in `run'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:2044:in `standard_exception_handling'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/lib/rake.rb:1974:in `run'
/usr/lib64/ruby/gems/1.8/gems/rake-0.8.4/bin/rake:31
/usr/bin/rake:19:in `load'
/usr/bin/rake:19

Answer 1

I believe I've found a practical non-intrusive method to get at the root of the problem. It works for me (Rails 2.3) :

As it happens Exception#blame_file! is invoked at some point in spite of the fact that it will fail and cause a new error, thus masking the original error.

So, open your FIRST initializer and add,

class Exception
  def blame_file!( file )
    puts "CULPRIT >> '#{file.to_s}' # #{self.to_s}"
  end
end

You'll get both the incriminated file and the original error message. That should be enough to pinpoint your problem.

Don't forget to remove the Exception initializer snippet.

Answer 2

The cause of this error is a double exception. Usually something in your code is crashing, which raises an initial exception. Then Rails' custom require attempts to keep the namespace clean by removing partially defined constants, which is the purpose of the new_constants_in method. The problem is that new_constants_in is not properly handling some particular construction somewhere within the code, I suspect due to mishandling of module namespaces or something (because ClassMethods is probably inside some module other than Object). In any case, I have not traced the error back to a Rails component or anything else, because frankly it's not worth the effort.

The solution (short of proposing something a little less invasive to Rails core) is a quick hack to figure out what raised the original exception. All you need to do is go to where Dependencies.new_constants_in is called and comment it out (there are a few places where this could be). So for example:

def require(file, *extras) #:nodoc:
  if Dependencies.load?
    Dependencies.new_constants_in(Object) { super }
  else
    super
  end
rescue Exception => exception  # errors from required file
  exception.blame_file! file
  raise
end

Comment out the new_constants_in stuff:

def require(file, *extras) #:nodoc:
#  if Dependencies.load?
#    Dependencies.new_constants_in(Object) { super }
#  else
    super
#  end
#rescue Exception => exception  # errors from required file
#  exception.blame_file! file
#  raise
end

Then you'll see your error straight away.