Rails - Jan 2008 - Redefining before_validation for an ActiveRecord model

Hi

I''m struggling more then a day now to try to write a simple plugin.

Let me explain what the plugin should do.
Consider a table called categories that looks like:

create_table :categories do |t|
     t.column :id,                 :integer
     t.column :categorizable_type, :string, :null => false
     t.column :categorizable_id,     :integer, :null => false, 
:references => nil
     t.column :category_name,       :integer, :null => false
end

with model:

class Category < ActiveRecord::Base
 belongs_to :categorizable, :polymorphic => true
 validates_presence_of :categorizable_type, :categorizable_id
end

Then for example a product is categorizable, table and model of product 
look like this:

create_table :products do |t|
     t.column :id,                 :integer
     t.column :code,            :string
end

class Product < ActiveRecord::Base
  has_many :categories, :as => :categorizable
end


So products are categorizable. Now I want to define a model 
product_categories that contains all categories for products.

The definition of product_categories is:
class ProductCategory < Category
    set_scope :categorizable_type, "Product"
end


Notice the ''set_scope'' definition. This is the plugin I am
writing
(called ScopedModel) and the idea is that when a model gets the 
set_scope definition, automatically the categorizable_type should be on 
''Product'' for all manipulations.
For example:

ProductCategory.find(:all) should result in:

select * from categories where categorizable_type=''Product''

ProductCategory.create!(category_name => ''some_name'')
should result in a
record being created where categorizable_type has value
''Product''.

So all operations on ProductCategory should result in making sure that 
everything is qualified with ''categorizable_type =
''Product''.
Until now I have been able to make it work for find, but I can''t let it
work for create, update ,...

My working plugin code until now is:

ActiveRecord::Base.send :include, ScopedModel

module ScopedModel
    def self.included(base)
       base.class_eval do
            class << self
                alias_method :orig_find_every, :find_every

                def set_scope(attribute,value)
                    @model_scope ||= {}
                    @model_scope[attribute] = value
                end

                # redefine find
                private
                def find_every(*args)
                    @find_option = Hash.new
                    @model_scope ||= {}
                    @model_scope.each_key do |attribute|
                        value = @model_scope[attribute]
                        @find_option[:conditions] = "#{attribute} = 
''#{value}''"
                    end
                    with_scope :find => (@find_option) do
                        orig_find_every(*args)
                    end
                end
            end
        end
    end
end

(I borrowed the code from the excellent HOWTO, see 
http://www.railsforum.com/viewtopic.php?id=682

So the find method automatically adds the scope qualification by adding 
it to the find of the class''s singleton class.
For the create, update, I want to use the ''before_validation''
method. If
I put next in my ProductCategory model:

def before_validation
  self.categorizable_type = ''Product''
end

Problem is that I should be able to define this before_validation in the 
plugin in the singleton class for product_category.
And I tried everything but nothing works. The code in the plugin should 
be something like:

alias_method :orig_before_validation, :before_validation
def before_validation
  @model_scope ||= {}
  @model_scope.each_key do |attribute|
     self.attribute = @model_scope[attribute]
  end
  orig_before_validation
end

Problem is that before_validation is mixed_in in ActiveRecord::Base and 
also that it should work on an instance and not at class_level, while my 
definition of set_scope is at (singleton) class level,

Hope that anyone can shed some light on this problem.

Thanks and Regards
Wim
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