How to extend DataMapper::Resource with custom method

Question

I have following code:

module DataMapper
  module Resource

   @@page_size = 25

   attr_accessor :current_page  
   attr_accessor :next_page
   attr_accessor :prev_page

  def first_page?
    @prev_page
  end

  def last_page?
    @next_page      
 end

  def self.paginate(page)
    if(page && page.to_i > 0)
      @current_page = page.to_i - 1
    else
      @current_page = 0
    end

    entites = self.all(:offset => @current_page  * @@page_size, :limit => @@page_size + 1)

    if @current_page > 0
      @prev_page = @current_page
    end

    if entites.size == @@page_size + 1
      entites.pop
      @next_page = (@current_page || 1) + 2
    end

    entites
  end
end

end

Then I have call of #paginate:

@photos = Photo.paginate(params[:page])

And getting following error:

application error
NoMethodError at /dashboard/photos/
undefined method `paginate' for Photo:Class

In Active record this concept works fine for me... I'am using JRuby for notice. What I'am doing wrong?

Answer 1

Andrew,

You can think of DataMapper::Resource as the instance (a row) and of DataMapper::Model as the class (a table). Now to alter the default capabilities at either the resource or the model level, you can either append inclusions or extensions to your model.

First you will need to wrap your #paginate method in a module. I've also added a probably useless #page method to show how to append to a resource in case you ever need to.

module Pagination
  module ClassMethods
    def paginate(page)
      # ...
    end
  end
  module InstanceMethods
    def page
      # ...
    end
  end
end

In your case, you want #paginate to be available on the model, so you would do:

DataMapper::Model.append_extensions(Pagination::ClassMethods)

If you find yourself in need of adding default capabilities to every resource, do:

DataMapper::Model.append_inclusions(Pagination::InstanceMethods)

Hope that helps!