From: Zach Dennis <zdennis@...>
Date: 2005-08-12T12:19:54+09:00
Subject: Re: Dynamically added methods

Yann Klis wrote:
> In fact, I'd like to mimic the behaviour of ActiveRecord.
> 
> If I have something like this:
> 
> class Base
> 
>   def initialize(attributes)
>     #do something clever with attributes
>   end
> 
> end
> 
> class MyObject < Base
> end
> 
> I'd like to be able to do that:
> 
> object = MyObject.new({"attribute1" => "value1", "attribute2" =>
> "value2"})
> val = object.attribute1
> 

To create methods on the class itself so all instances of that class get 
the method you can do:

class A
   def initialize( args=nil )
     if args.is_a? Hash
       args.each { |key,val| self.class.send( :define_method, key, proc 
{ val } ) }
     end
   end
end

a = A.new( :zzz => "testing" )
puts a.zzz
b = A.new
puts b.zzz

To create methods on the instance you have created only you can do:

class A
   def initialize( args=nil )
     if args.is_a? Hash
       s = class << self; self; end
       args.each { |key,val| s.send( :define_method, key, proc { val } ) }
     end
   end	
end

a = A.new( :zzz => "testing" )
puts a.zzz
b = A.new
puts b.zzz #this will throw an error,because method zzz doesn't exist on 
this instance

Note, that I think the second example above is bad design. We are 
relying on an instance of A to give the class A extra methods for all 
instances. We should instead rely on a singleton method for class A to 
handle this task. (keep reading)

The above two solutions don't take into consideration if a method 
already exists though, so you could easily overwrite an existing method. 
Another way to do this is by overriding the method missing for your 
class which won't override existing methods. The below will handle 
things for instances only, like the first example:

class A
   def initialize( args={} )
     @args = args if args.is_a? Hash
   end

   def method_missing( method )
     @args[method] || super
   end
end

puts A.new( :a => "z" )  # prints out "z"
A.new.a  #throws an error because :a wasn't passed in to this object


And you can alter that to add methods for the class also so all 
instances get the methods, although above in the second example I think 
we used bad designed. So here we fix it by using a singleton method 
'add_attributes':

class A
   def self.add_attributes( attributes )
     @@attributes = attributes
   end

   def initialize( args={} )
     @args = args if args.is_a? Hash
   end

   def method_missing( method )
     @@attributes[method] || super
   end
end

A.add_attributes( :attr1 => "attribute 1", :attr2 => "attribute 2" )
puts A.new.attr1
puts A.new.attr2

I hope this helps!

Zach