Tricky metaprogramming

Rails
1

Hi
This is a tricky one I think.I have a Rails plugin that handles
collections. For example I want to be able to write the following in a
model:

class User < ActiveRecord::Base
my_class_method :collections => [:pages, :chapters, :books]
end

With only the relevant pieces left and with neutral names the module
file looks like this:

module MyModule1
module MyModule2 #:nodoc:
module MyModule3 #:nodoc:

  def self.included(mod)
    mod.extend(ClassMethods)
  end

  module ClassMethods
    def my_class_method(*args)
      @collections = args[0][:collections]

      extend

MyModule1::MyModule2::MyModule3::CollectionSingletonMethods
include
MyModule1::MyModule2::MyModule3::CollectionInstanceMethods
end
end

  module CollectionSingletonMethods
  end

  module CollectionInstanceMethods
    def self.included(base)
      base.class_eval do
        @collections.each do |collection|
          define_method collection do
            # Do something useful for the collection here
          end
        end
      end
    end
  end
end

end
end

This means I can create one method per entry in the collection to be
called like user_instance.pages, user_instance.chapters and so on. So
far so good.

But now I want to define one method that does something for every
collection. So this call:

user_instance.do_something_for_all

would call a method that if it was hardcoded would look like something
like this:

class User
def so_something_for_all
@pages = []
@chapters = []
@books = []
end
end

Does anyone know how to accomplish this in the framework above. I
would be extremely happy for all input in this, references to texts
about it is very welcome.

Kindest regards

Erik L.

2

On 15 Aug 2008, at 19:22, Erik L. wrote:

@books = []
end
end

Does anyone know how to accomplish this in the framework above. I
would be extremely happy for all input in this, references to texts
about it is very welcome.

is it any more complicated than

def do_something_for_all
self.class.collections.each {|collection| yield
self.send(collection)}
end

or have I missed something? (assuming you have a class method that
returns the names of the collections and an accessor method for each
collection)

Fred

3

Hi Fred and thanks for taking the time to read through this long
question.

I think for this particular example I can build a method in the way
you describe. I got so intertwined in Procs and metaprogramming that
this solution eluded me.

But for arguments sake, assume I wanted to construct the whole method
body in advance, for example as a string and create a method from
that. Would that be possible? Maybe put the whole define_method
thingie in a string and run eval on that?

Kindest regards

Erik

4

On 15 Aug 2008, at 22:55, Erik L. wrote:

that. Would that be possible? Maybe put the whole define_method
thingie in a string and run eval on that?

I’m not quite sure what you’re getting at but you ca do pretty much
anything you want with the string form of class_eval.

Fred

5

Hi Fred

Yes, the string form of class_eval did just the trick.

Many thanks for the help. Priceless.

Regards

Erik