polymorphic

gistfile1.txt

class Comment < ActiveRecord::Base
  belongs_to :commentable, :polymorphic => true
end

class Article < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

class Photo < ActiveRecord::Base
  has_many :comments, :as => :commentable
  #...
end

class Event < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

# comments_controller
def index
  @commentable = find_commentable
  @comments = @commentable.comments
end

def create
  @commentable = find_commentable
  @comment = @commentable.comments.build(params[:comment])
  if @comment.save
    flash[:notice] = "Successfully created comment."
    redirect_to :id => nil
  else
    render :action => 'new'
  end
end

private

def find_commentable
  params.each do |name, value|
    if name =~ /(.+)_id$/
      return $1.classify.constantize.find(value)
    end
  end
  nil
end

# routes.rb
map.resources :articles, :has_many => :comments
map.resources :photos, :has_many => :comments
map.resources :events, :has_many => :comments