Rails - Mar 2005 - I have a dream for easier meta-programming in Ruby

I really love things in Ruby on Rails like:

class Blog < ActiveRecord::Base
	has_many :comments
end

Particularly, I love the fact that in Ruby, you can do meta-programming 
to define has_many in a way that extends the class accordingly. This 
allows you to build a language for the development environment you are 
working with. When I found out that has_many was simply a function call 
that added functions and relations to the class, I went to irb and 
typed:

class Test
	attr_reader :foo

	def something(x)
		@foo = x
	end

	something "bar"
end

a = Test.new
puts a.foo

Fully expecting to see "bar". Instead, I saw "NoMethodError:
undefined
method `something'' for Test:Class". Exploring Rails source code, I
found that this meta-programming feature of Ruby is not as easy to 
create as I thought. My dream is that Ruby could do this. Is there 
anyone out there that knows of a movement I could join to incorporate 
this behavior, or should I start this movement?

-Lucas
http://rufy.com/

On Sunday, March 27, 2005, 1:35:44 PM, Lucas wrote:
> class Test
>   attr_reader :foo
>   def something(x)
>     @foo = x
>   end
>   something "bar"
> end
> a = Test.new
> puts a.foo
> Fully expecting to see "bar". Instead, I saw "NoMethodError:
undefined
> method `something'' for Test:Class".
What you''ve done is equivalent to

  Test.something "bar"

Whereas what you _need_ to do is:

  a = Test.new              # You got this bit right
  a.something "bar"
  puts a.foo                # And this bit
> Exploring Rails source code, I found that this meta-programming
> feature of Ruby is not as easy to create as I thought. My dream is
> that Ruby could do this. Is there anyone out there that knows of a
> movement I could join to incorporate this behavior, or should I
> start this movement?
You need to work out exactly what you want to do.  Provide code
examples and expected outputs.  Explain them.  The example above, for
instance, is not very good.  It does show that some things about Ruby
are confusing at first, and maybe you can make some suggestions to
improve that situation.

Maybe take some of the Rails code and demonstrate how you think it
should be able to be written.

Cheers,
Gavin

I''m a little lost as to what you are hoping from with your example.
Here''s a
semi-useful example of meta programming in Ruby: a naive interceptor 
implementation:

module Interceptable
  def intercept(selector, options={})
    intercepted_selector = "intercepted_#{selector}"
    unless instance_methods.include?(intercepted_selector)
      self.send(:alias_method, intercepted_selector, selector)
      interceptors = ((@interceptors ||= {})[selector] ||= {:before => 
[], :after => []})
      self.send(:define_method, selector) do |*args|
        args = interceptors[:before].inject(args) {|args, intc| 
intc.call(self, *args) || args}
        result = self.send(intercepted_selector, args)
        interceptors[:after].inject(result) {|r, intc| intc.call(r) || r}
      end
      interceptors[options[:when] || :before] << Proc.new
    end
  end
end

class A
  extend Interceptable
  def square(x) x * x end
end

puts A.new.square(10) # => 100
A.intercept(:square) {|a, x| x + 1}
puts A.new.square(10) # => 121


This example doesn''t illustrate the typical override of
def append_features(base)
  super(base)
  base.extend(ClassMethods)
end

which I suspect might be your gripe wrt. meta programming.

Personally, I wouldn''t mind a core extension of
module AutoExtend
  def append_features(base)
    super(base)
    base.extend(self::ClassMethods) if self.const_defined? :ClassMethods
  end
end
class Module ; include AutoExtend end

-- 

Nicholas Seckar aka. Ulysses

Lucas Carlson wrote:
> Particularly, I love the fact that in Ruby, you can do meta-programming 
> to define has_many in a way that extends the class accordingly. This 
> allows you to build a language for the development environment you are 
> working with. When I found out that has_many was simply a function call 
> that added functions and relations to the class, I went to irb and typed:
> 
> class Test
>     attr_reader :foo
> 
>     def something(x)
>         @foo = x
>     end
This ought to be

       def self.something(x)
           @foo = "x"
       end
>     something "bar"
> end
Why is it this way?

Inside a Class or Module definition  def foo() end  creates an instance 
method with the name foo. All instances of the Class you are defining or 
all instances of classes including the Module you are defining will then 
automatically have that method. The Class or Module itself won''t.

So what you need to do is to create a method on the Class or Module 
*itself*. This can be accomplished via the  def self.foo() end  syntax 
which creates a method in the singleton class of your Class. (Singleton 
classes store the methods for individual objects.)

Note that the above will only make the  something "bar"  construct 
available to Test and its subclasses. If you want to introduce a meta 
programming method to all Module and Class definitions you can do it 
like this:

class Module
   def foo() ... end
end

Note that this time it''s not called "self.foo" as all classes
and
modules are instances of Module. (classes are instances of Class which 
inherits from Module.)

Perhaps you will see the connection easier if you try out the following 
sample code in IRB:

irb(main):001:0> obj = "foo"
=> "foo"
irb(main):002:0> def obj.reverse() "ought to be
''oof''" end
=> nil
irb(main):003:0> obj.reverse
=> "ought to be ''oof''"
irb(main):004:0> "hello".reverse
=> "olleh"

This creates the singleton method  reverse  for only one object. The 
same was happening above when we created the  something  method for only 
one object which happened to be the class Test.

>This ought to be
>
>      def self.something(x)
>          @foo = "x"
>      end
>
>     something "bar"
> end
While this removes the "undefined method" error, it still
doesn''t do
what the original poster intended.  @foo is an instance variable so it
won''t get set until the class is instantiated.  You can use @@foo in
this example and it will work, but @@foo will be the same across all
instances of your class.

I think what the original poster was asking--which hasn''t really been
answered--is how would one go about defining Rails-like functionality
like "has_many" or "validates_uniqueness_of" etc. in classes
of one''s
own.  The Rails code is a little overwhelming for someone who doesn''t
yet breathe Ruby like air.

-dave

> I think what the original poster was asking--which hasn''t really
been
> answered--is how would one go about defining Rails-like functionality
> like "has_many" or "validates_uniqueness_of" etc. in
classes of one''s
> own.  The Rails code is a little overwhelming for someone who
doesn''t
> yet breathe Ruby like air.
Totally agree David.  I''m most interested in how the has_many
functionality etc. is acheived.  Sadly I''m still being weaned off VB
air at the moment !





On Sun, 27 Mar 2005 07:13:19 -0600, David Adams
<daveadams-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> >This ought to be
> >
> >      def self.something(x)
> >          @foo = "x"
> >      end
> >
> >     something "bar"
> > end
> 
> While this removes the "undefined method" error, it still
doesn''t do
> what the original poster intended.  @foo is an instance variable so it
> won''t get set until the class is instantiated.  You can use @@foo
in
> this example and it will work, but @@foo will be the same across all
> instances of your class.
> 
> I think what the original poster was asking--which hasn''t really
been
> answered--is how would one go about defining Rails-like functionality
> like "has_many" or "validates_uniqueness_of" etc. in
classes of one''s
> own.  The Rails code is a little overwhelming for someone who
doesn''t
> yet breathe Ruby like air.
> 
> -dave
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

Woops, I can see that my second dream is already a reality, I should 
have tried that in IRB before I talked. :)

-Lucas
http://www.rufy.com/

I am getting an error using "Pagination" with the new rails 0.11.1
version.

I have a very simple case of pagination in my program. In my controller I 
have:
paginate :films

and in my view I have:
<%= pagination_links(@film_pages) %>

and I get the following error:
NameError in Films#list
(eval):1:in `paginator_and_collection_for'': uninitialized constant 
ActionController::Pagination::Film

This code worked before in 0.11.0. Did someone else experience the same?

Pb.

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David Adams wrote:
>>This ought to be
>>
>>     def self.something(x)
>>         @foo = "x"
>>     end
>>
>>    something "bar"
>>end
> 
> 
> While this removes the "undefined method" error, it still
doesn''t do
> what the original poster intended.  @foo is an instance variable so it
> won''t get set until the class is instantiated.  You can use @@foo
in
> this example and it will work, but @@foo will be the same across all
> instances of your class.
No, this is an instance variable on the class which is perfectly valid.

It can be accessed via Test.instance_variable_get(:@foo) in the above 
case. You can also create accessors for it like this:

   class << self
     attr_accessor :foo
   end
> I think what the original poster was asking--which hasn''t really
been
> answered--is how would one go about defining Rails-like functionality
> like "has_many" or "validates_uniqueness_of" etc. in
classes of one''s
> own.  The Rails code is a little overwhelming for someone who
doesn''t
> yet breathe Ruby like air.
I''d do it exactly like this. If you define the above self.something in
a
common base class it will work the way it is intended to do.

Am I overseeing any problems?

Hi

I''m having trouble using the new "Yellow fade" effect (newly
upgraded
Rails 0.11.1):

View:

<p>
<%= link_to_function ''Highlight text'',
"Effect.Highlight(''paragraph'')" %>
</p>
<p id=''paragraph''>Text to highlight</p>


Clicking ''Highlight text'' generates the following error
(Firefox 1.0.2):

Error: this.initialize has no properties
Source File: http://blackbird:3000/javascripts/prototype.js
Line: 15

Also get a Javascript error in IE6.

I must be missing something, any help appreciated.


Cheers, Stuart
--
Stuart Rackham

> <%= link_to_function ''Highlight text'', "new 
> Effect.Highlight(''paragraph'')" %>
Needs the new for initialization.
--
David Heinemeier Hansson,
http://www.basecamphq.com/   -- Web-based Project Management
http://www.rubyonrails.org/  -- Web-application framework for Ruby
http://www.loudthinking.com/ -- Broadcasting Brain

But he''s trying to set the instance variable in the class definition. 
That doesn''t take.

class S
  attr_reader :zzz
  def self.rails_like_attribute(x)
    @zzz = x
  end
end

class T < S
  rails_like_attribute ''testing''
end

puts T.new.zzz

This outputs ''nil'', when the intent was to output
''testing''.

If you swap things around to use a class variable, you get a different problem:

class S
  def zzz
    @@zzz
  end

  def self.rails_like_attribute(x)
    @@zzz = x
  end
end

class T < S
  rails_like_attribute ''one''
end

puts T.new.zzz

class Q < S
  rails_like_attribute ''two''
end

puts Q.new.zzz
puts T.new.zzz


That code outputs:
one          > first instance of T
two          > first instance of Q
two          > instance of T created after def of Q

So, my question, at least, is: how do you set this up so that each
subclass can have a different setting using only lines like
"rails_like_attribute ''three''"?  Rails does this
extensively, but I
haven''t seen any documentation of exactly how it works.  PickAxe has
not enlightened me.  Digging through the ActiveRecord code is helpful
in some ways but confusing in others.  Can anyone give a simple
example of how to do what I want (and presumably help the original
poster)?

-dave

On Sun, 27 Mar 2005 23:12:04 +0200, Florian Groß
<florgro-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> David Adams wrote:
> 
> >>This ought to be
> >>
> >>     def self.something(x)
> >>         @foo = "x"
> >>     end
> >>
> >>    something "bar"
> >>end
> >
> >
> > While this removes the "undefined method" error, it still
doesn''t do
> > what the original poster intended.  @foo is an instance variable so it
> > won''t get set until the class is instantiated.  You can use
@@foo in
> > this example and it will work, but @@foo will be the same across all
> > instances of your class.
> 
> No, this is an instance variable on the class which is perfectly valid.
> 
> It can be accessed via Test.instance_variable_get(:@foo) in the above
> case. You can also create accessors for it like this:
> 
>    class << self
>      attr_accessor :foo
>    end
> 
> > I think what the original poster was asking--which hasn''t
really been
> > answered--is how would one go about defining Rails-like functionality
> > like "has_many" or "validates_uniqueness_of" etc.
in classes of one''s
> > own.  The Rails code is a little overwhelming for someone who
doesn''t
> > yet breathe Ruby like air.
> 
> I''d do it exactly like this. If you define the above
self.something in a
> common base class it will work the way it is intended to do.
> 
> Am I overseeing any problems?
> 
> _______________________________________________
> Rails mailing list
> Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>

David Heinemeier Hansson wrote:
>> <%= link_to_function ''Highlight text'', "new 
>> Effect.Highlight(''paragraph'')" %>
> 
> 
> Needs the new for initialization.
Thanks, it now works like a charm!


Cheers, Stuart

David Adams wrote:
> 
> So, my question, at least, is: how do you set this up so that each
> subclass can have a different setting using only lines like
> "rails_like_attribute ''three''"?  Rails does
this extensively, but I
> haven''t seen any documentation of exactly how it works.  PickAxe
has
> not enlightened me.  Digging through the ActiveRecord code is helpful
> in some ways but confusing in others.  Can anyone give a simple
> example of how to do what I want (and presumably help the original
> poster)?
> 
> -dave
> 
A simple example

# Meta-programming
# Add a public array property to an object instance

class Meta
   def self.has_many( sym )
     attr_tmp = "def  #{sym.to_s} \n"
     attr_tmp << " @#{sym.to_s} = [] unless @#{sym.to_s}\n"
     attr_tmp << " @#{sym.to_s}; end"
     eval ( attr_tmp  )
   end
end


class Foo < Meta
   has_many :foo
   has_many :baz
end


f = Foo.new

# See what methds the new object has ...
puts (f.methods - Object.methods  ).inspect # ["foo", "baz"]

f.foo.push "X"
f.baz << "hello"

p f.foo.first # "X"
p f.baz       # ["hello"]



James Britt

David Adams wrote:
> But he''s trying to set the instance variable in the class
definition.
> That doesn''t take.
Which is not exactly possible as at definition time you do not yet have 
access to your instances. But you can set up instance methods already.
> class S
>   attr_reader :zzz
>   def self.rails_like_attribute(x)
>     @zzz = x
>   end
> end
> 
> class T < S
>   rails_like_attribute ''testing''
> end
> 
> puts T.new.zzz
> 
> This outputs ''nil'', when the intent was to output
''testing''.
> 
> If you swap things around to use a class variable, you get a different
problem:
> 
> [Snip, don''t do that then.]
> 
> So, my question, at least, is: how do you set this up so that each
> subclass can have a different setting using only lines like
> "rails_like_attribute ''three''"?  Rails does
this extensively, but I
> haven''t seen any documentation of exactly how it works.  PickAxe
has
> not enlightened me.  Digging through the ActiveRecord code is helpful
> in some ways but confusing in others.  Can anyone give a simple
> example of how to do what I want (and presumably help the original
> poster)?
Perhaps you are looking for .define_method then:

class S
   def self.rails_like_attribute(x)
     define_method(:zzz) { x }
   end
end

But the class instance variable approach works as well:

class S
   class << self
     def rails_like_attribute(x)
       @zzz = x
     end

     attr_reader :zzz
   end

   def zzz() self.class.zzz end
end

Is this what was wanted then? It all pretty much looks the same for me, 
unfortunately.

I tried something similar today, and wanted to put the code into a
module. After digging through the rails source, here is a working test
case that I created. The trick is in the append_features call:

------------------------------------
module SomeModule

  def bark
    puts "woof"
  end

  def self.append_features(base)
    super
    base.extend(ClassMethods)
  end

  module ClassMethods
    def createsomething(attrname)
      define_method(attrname) { "Method #{attrname} called" }
    end
  end
end

class SomeClass
  include SomeModule
  createsomething   :meow
end

a = SomeClass.new
a.bark
a.meow
------------------------------------

Hi,

I am getting an error using "Pagination" with the new rails 0.11.1
version.

I have a very simple case of pagination in my program. In my controller I 
have:
paginate :films

and in my view I have:
<%= pagination_links(@film_pages) %>

and I get the following error:
NameError in Films#list
(eval):1:in `paginator_and_collection_for'': uninitialized constant 
ActionController::Pagination::Film

This code worked before in 0.11.0. Did someone else experience the same?

Pb.

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On Sun, 27 Mar 2005 20:49:09 +0000, Pranav Bihari
<pbihari-PkbjNfxxIARBDgjK7y7TUQ@public.gmane.org>
wrote:
> and I get the following error:
> NameError in Films#list
> (eval):1:in `paginator_and_collection_for'': uninitialized constant
> ActionController::Pagination::Film
> 
> This code worked before in 0.11.0. Did someone else experience the same?
I''m getting the same error, just upgrading from rails 0.10.1 to
0.11.1. I deleted the old pagination_helper.rb file and did a ''rails .
--skip''.

What are we missing?

-- 
Urban Artography
http://artography.ath.cx

On Mon, 28 Mar 2005 21:51:36 -0700, Rob Park
<rbpark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> What are we missing?
Oh, duh. Had to add ''model :photo'' to the beginning of the
class
definition in the controller. (Pranav, you''d have to add
''model :film''
most likely).

Does anybody know why this change is necessary?

-- 
Urban Artography
http://artography.ath.cx

Yes, I realized that we need to include the model explicitly. But I
don''t
see why.

Also, the :params functionality still doesn''t work for me even in
0.11.1. I
follow the instructions on the Twiki page on creating a paginate partial and 
using it in my view, while passing additional query arguments.

Can someone check the new paginator functionality with 0.11.1 and let us 
know if it is working as it should?

Thanks,
Pb
>From: Rob Park <rbpark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>Reply-To: Rob Park <rbpark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
>To: rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
>Subject: Re: [Rails] pagination error
>Date: Mon, 28 Mar 2005 21:54:03 -0700
>
>On Mon, 28 Mar 2005 21:51:36 -0700, Rob Park
<rbpark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > What are we missing?
>
>Oh, duh. Had to add ''model :photo'' to the beginning of the
class
>definition in the controller. (Pranav, you''d have to add
''model :film''
>most likely).
>
>Does anybody know why this change is necessary?
>
>--
>Urban Artography
>http://artography.ath.cx
>_______________________________________________
>Rails mailing list
>Rails-1W37MKcQCpIf0INCOvqR/iCwEArCW2h5@public.gmane.org
>http://lists.rubyonrails.org/mailman/listinfo/rails
_________________________________________________________________
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On Mar 28, 2005, at 8:54 PM, Rob Park wrote:
> On Mon, 28 Mar 2005 21:51:36 -0700, Rob Park
<rbpark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> What are we missing?
>
> Oh, duh. Had to add ''model :photo'' to the beginning of
the class
> definition in the controller. (Pranav, you''d have to add
''model :film''
> most likely).
>
> Does anybody know why this change is necessary?
I''m seeing the same symptom (paginate worked in 0.11.0, not in 0.11.1),
but adding the ''model'' line doesn''t seem to help
anything.  Adding a
''require "model"'' to the top of my controller file
fixes the problem
for *1* page view after restarting the app; after that, it''s broken 
again.  I added a bunch of debugging code to 
paginator_and_collection_for, and when it runs, none of my model 
classes are defined.  The controller classes are present, but the 
models are not.  That''s why we''re seeing the
''unitialized constant
ActionController::Pagination::<model>, because the model class
doesn''t
exist at that point.

I''ve tried to follow the load/unload/reload code, and I don''t
have a
clue what it''s doing.

This is on OS X with Ruby 1.8.2, built from source last week, using 
Rails 0.11.1 from gems.


Scott

On Mar 29, 2005, at 3:54 PM, Scott Laird wrote:
>
> On Mar 28, 2005, at 8:54 PM, Rob Park wrote:
>
>> On Mon, 28 Mar 2005 21:51:36 -0700, Rob Park
<rbpark-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>> What are we missing?
>>
>> Oh, duh. Had to add ''model :photo'' to the beginning
of the class
>> definition in the controller. (Pranav, you''d have to add
''model :film''
>> most likely).
>>
>> Does anybody know why this change is necessary?
>
> I''m seeing the same symptom (paginate worked in 0.11.0, not in 
> 0.11.1), but adding the ''model'' line doesn''t
seem to help anything.
> Adding a ''require "model"'' to the top of my
controller file fixes the
> problem for *1* page view after restarting the app; after that,
it''s
> broken again.  I added a bunch of debugging code to 
> paginator_and_collection_for, and when it runs, none of my model 
> classes are defined.  The controller classes are present, but the 
> models are not.  That''s why we''re seeing the
''unitialized constant
> ActionController::Pagination::<model>, because the model class
doesn''t
> exist at that point.
>
> I''ve tried to follow the load/unload/reload code, and I
don''t have a
> clue what it''s doing.
>
> This is on OS X with Ruby 1.8.2, built from source last week, using 
> Rails 0.11.1 from gems.
Someone asked me to follow this up with details from my controller:

class GroupsController < ApplicationController
   model :group  # added as part of testing 0.11.1, wasn''t needed in 
0.11.0
   layout :standard-layout

   def list
     order=set_order

     @paginator, @items=paginate(:groups, :order_by=>order, 
:per_page=>15)
   end

   def show
     @group=Group.find(@params[:id])
   end

   ...
end

This worked flawlessly with 0.11.0, but dies every time I try to visit 
/groups/list with a error like this:

  NameError in  Groups#list

  (eval):1:in `paginator_and_collection_for'': uninitialized constant 
ActionController::Pagination::Group

     app/controllers/groups_controller.rb:15:in `list''
     ./script/server:48

Now, adding ''require "group"'' to the top of the file
makes this work
*once*.  Hitting reload gives me the error again, and keeps giving me 
the error until I restart webrick.  I added some debugging code to 
paginator_and_collection_for, and it shows that *none* of my model 
classes exist when it''s called.  I determined this by walking 
ObjectSpace and printing the name of all Class objects.

Strangely, though, the ''show'' method works flawlessly every
time.

I tried creating a new Rails project and then copying over my models, 
controllers, and views from 0.11.0, just to see if I''d corrupted 
environment.rb or something, but that didn''t make any
difference--it''s
still broken.

Anyone have any suggestions?


Scott

On Mar 30, 2005, at 10:11 AM, Scott Laird wrote:
> Someone asked me to follow this up with details from my controller:
>
> class GroupsController < ApplicationController
>   model :group  # added as part of testing 0.11.1, wasn''t needed
in
> 0.11.0
>   layout :standard-layout
>
>   def list
>     order=set_order
>
>     @paginator, @items=paginate(:groups, :order_by=>order, 
> :per_page=>15)
>   end
>
>   def show
>     @group=Group.find(@params[:id])
>   end
>
>   ...
> end
>
> This worked flawlessly with 0.11.0, but dies every time I try to visit 
> /groups/list with a error like this:
>
>  NameError in  Groups#list
>
>  (eval):1:in `paginator_and_collection_for'': uninitialized
constant
> ActionController::Pagination::Group
>
>     app/controllers/groups_controller.rb:15:in `list''
>     ./script/server:48
>
> Now, adding ''require "group"'' to the top of the
file makes this work
> *once*.  Hitting reload gives me the error again, and keeps giving me 
> the error until I restart webrick.  I added some debugging code to 
> paginator_and_collection_for, and it shows that *none* of my model 
> classes exist when it''s called.  I determined this by walking 
> ObjectSpace and printing the name of all Class objects.
>
> Strangely, though, the ''show'' method works flawlessly
every time.
>
> I tried creating a new Rails project and then copying over my models, 
> controllers, and views from 0.11.0, just to see if I''d corrupted 
> environment.rb or something, but that didn''t make any
difference--it''s
> still broken.
Ugh, brown-paper bag time.  I was editing one version of the code while 
running a different version; that explains why adding ''model''
didn''t
help.  Once I fixed that, I found that either of two things would fix 
the problem:

   1.  Adding the model line, as mentioned a couple messages up-thread.
   2.  Adding a reference to the model *before* calling the Pagination 
code.  So adding ''Group.find(1)'' before the call to paginate
also fixed
things.


Scott