On 24 Dec 2008, at 02:26, Sarah Allen wrote:
Matt Wynne wrote:
ActiveRecord::Validations uses a trick that's used widely in rails,
and is arguably[1] becoming a Ruby idiom, apparently.
This is the interesting bit, around line #275:
def self.included(base) # :nodoc:
base.extend ClassMethods
So basically that means that when the class ActiveRecord::Base
includes the module ActiveRecord::Validations, that module extends
the
thing which included it (in this case ActiveRecord::Base) with the
class methods defined in ActiveRecords::Validations::ClassMethods.
Does that make sense?
um, no. You say that the class ActiveRecord::Base includes the module
ActiveRecord::Validations, but I don't see that in the code I'm
looking
at here:
http://dev.rubyonrails.org/browser/branches/2-0-stable/activerecord/lib/active_record/base.rb
You almost got it. Try here instead:
http://dev.rubyonrails.org/browser/branches/2-0-stable/activerecord/lib/active_record.rb
If I assume that I'm looking at the wrong code or it happens thru some
magic then you can make a module which will add its class methods to
the
class which includes it?
Yes, that's basically what the self.included trick does. As I say,
it's debatable whether this is a neat trick or a horrible hack. If you
were reading the code that included the module,. might have only
expected that would add instance methods, rather than class methods as
well.
Is that some kind of trick to create multiple
inheritance?
Not really in that particular case. Ruby modules generally allow you
to do multiple inheritance by 'mixing in' behaviour to classes, but
that would be when you use 'include' to bring the methods defined in
the module into the class itself.
Is there any way without digging thru source to know which
class include methods from other classes or do y'all just get so cozy
with every Rails class you use that you "just know?"
If you have hold of a class in ruby, there are ways to ask it what
modules / classes it inherits from, but I can't remember them off the
top of my head, sorry. Try messing around in irb for this kind of thing.
Matt Wynne
http://blog.mattwynne.net
http://www.songkick.com
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