Rails - Jan 2007 - Why Module/Class/Object are redefined in Active Support?

Recently i have decided to read sources of rails.In the source tree of
Active Support, I found in the dependencies.rb that Module/Class/Object
are redefined.

Can these keywords in ruby be redefined? And what are the main purposes
to do that?And what''s more,since Module/Class/Object are redefined,then
all the keywords emerged in the sources of activecontroller activeview
activerecord use the definition in dependencies.rb,is it right?

class Module #:nodoc:
  # Rename the original handler so we can chain it to the new one
  alias :rails_original_const_missing :const_missing

  # Use const_missing to autoload associations so we don''t have to
  # require_association when using single-table inheritance.
  def const_missing(class_id)
    Dependencies.load_missing_constant self, class_id
  end

  def unloadable(const_desc = self)
    super(const_desc)
  end

end

class Class
  def const_missing(class_id)
    if [Object, Kernel].include?(self) || parent == self
      super
    else
      begin
        begin
          Dependencies.load_missing_constant self, class_id
        rescue NameError
          parent.send :const_missing, class_id
        end
      rescue NameError => e
        # Make sure that the name we are missing is the one that caused
the error
        parent_qualified_name = Dependencies.qualified_name_for parent,
class_id
        raise unless e.missing_name? parent_qualified_name
        qualified_name = Dependencies.qualified_name_for self, class_id
        raise NameError.new("uninitialized constant
#{qualified_name}").copy_blame!(e)
      end
    end
  end
end

class Object #:nodoc:

  alias_method :load_without_new_constant_marking, :load

  def load(file, *extras)
    Dependencies.new_constants_in(Object) { super(file, *extras) }
  rescue Exception => exception  # errors from loading file
    exception.blame_file! file
    raise
  end

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

  # Mark the given constant as unloadable. Unloadable constants are
removed each
  # time dependencies are cleared.
  #
  # Note that marking a constant for unloading need only be done once.
Setup
  # or init scripts may list each unloadable constant that may need
unloading;
  # each constant will be removed for every subsequent clear, as opposed
to for
  # the first clear.
  #
  # The provided constant descriptor may be a (non-anonymous) module or
class,
  # or a qualified constant name as a string or symbol.
  #
  # Returns true if the constant was not previously marked for
unloading, false
  # otherwise.
  def unloadable(const_desc)
    Dependencies.mark_for_unload const_desc
  end

end

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> Can these keywords in ruby be redefined? And what are the main purposes
> to do that?
They''re not redefined.  In ruby, a class/module is never
''closed'' -
i.e. you can just add new methods and override old ones.  Some people 
call this ''monkey patching'' and act like it''s a bad
thing.  To
rubyists, it''s a powerful feature.  Rails extends and modifies the 
core of ruby to add the features that we know and love.  For example, 
if I wanted to add a method to all ActiveRecord objects for my 
application, I could just open up ActiveRecord and define it:

class ActiveRecord:Base
  def self.number_of_attributes
     @attributes.size
  end
end

I could then use this method on any of my models:
Person.number_of_attributes -> 3

Hope that helps,

Steve


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Thanks,it is because of my chaotic concept of ruby lang.


Fred

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