page.rb

class Page < ActiveRecord::Base                                                                                                                                                                                     
  attr_accessible :title                                                                                                                                                                                            
                                                                                                                                                                                                                    
  has_many :page_widgets                                                                                                                                                                                            
  has_many :widgets, through: :page_widgets                                                                                                                                                                         
end 

page_widget.rb

class PageWidget < ActiveRecord::Base  
  attr_accessible :height, :offset_x, :offset_y, :width
                                                                                                                                                                             
  belongs_to :page                                                                                                                                                                                                  
  belongs_to :widget
end

widget.rb

class Widget < ActiveRecord::Base
  attr_accessible :content, :title

  has_many :page_widgets
  has_many :pages, through: :page_widgets

  def put_on page
    pages << page
    # return new page_widget here
  end
end