#154 Polymorphic Association (revised)

Is this not a good solution due to no foreign key references?

There is a problem with polymorphic associations if the polymorphic side has Single Table Inheritance: then it will only list the base class as the type.

polymorphic has_many is much more useful. that way THE SAME photo can belong to many different models.

Thanks Ryan! And I picked up another text mate tip, CTRL + Shift + H (create partial from template). Actually, do you reckon that could make for a good topic? TextMate tips?

Ryan: to get the type of commentable tpye and ID, I use the follow code:

def get_commentable
    @commentable = params[:commentable].classify.constantize.find(commentable_id)
end

def commentable_id
  params[(params[:commentable].singularize + "_id").to_sym]
end

and append to the nested comment resource this

, :defaults => { :commentable => 'picture' }

(when commenting on picture)

This way, the only thing you have to remember when adding more commentable is to add the default param to the route and you are done ;-)

Unfortunately the official Rails api is misleading when describing how to use url_for and consequently link_to with nested routes. " If you have a nested route, such as admin_workshop_path you’ll have to call that explicitly (it’s impossible for url_for to guess that route)." This isn't the case since you could use url_for([:admin, Workshop.new]) or similar. All you need to do is provide a symbol or an object that's above the target wrapped in an array. Thanks Ryan for pointing this out.

Ryan,

I think the load_commentable before filter used in this RailsCasts episode is somewhat brittle and, probably too complicated to be used as an example, especially when demonstrating this feature to new users.

I would rather go with a simpler alternative. Something like:

def load_commentable
  @commentable = if params[:article_id]
    Article.find(params[:article_id])
  elsif params[:photo_id]
    Photo.find(params[:photo_id])
  elsif params[:event_id]
    Event.find(params[:event_id])
  end
end

Or even more verbose:

def load_commentable
  if params[:article_id]
    @commentable = Article.find(params[:article_id])
  elsif params[:photo_id]
    @commentable = Photo.find(params[:photo_id])
  elsif params[:event_id]
    @commentable = Event.find(params[:event_id])
  end
end

I see several benefits in this kind of code:

• Code is less clever, which is a good thing for controller code.

• Filter code is decoupled from changes to routes. For example, if a scope or namespace was added or the path option was used to produce easier to read routes, you would not need to change filter code or tests.

• Scopes can be added independently to each find call. Articles could be published and events active, without the need for aliases.

• Code produces no temporary objects nor relies on string processing which, over time, add up to app execution time.

• Tests are easier to follow and coverage easier to check.

I would also like to mention that, in real world, some of the most popular uses of polymorphic associations, like comments, are a bad pattern.

In this example, article comments will never be queried as event comments nor -most probably- listed together and, they even might end up having very different requirements. In Ruby, behavior can always be shared using modules.

Still, there are some valid uses for polymorphic associations but, way less than what one would think of first.

P.S. thanks so much for your work and contribution! I admit this is one of the very few cases where I don't agree with the code presented. :-)

Just put :comment_type => :article, or :events, :photo in the routes as a parameter inside of the resources block and then setup a before filter to classify that value (params[:comment_type]).

I have to agree that this cast was timed correctly for me as i was looking into something that could benefit from a polymorphic relation.
the other thing which i found useful is this code from stackoverflow, which shows how to set up a has many through polymorphic table

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :grouper, :source_type => 'Person'
  has_many :aliens, :through => :widget_groupings, :source => :grouper, :source_type => 'Alien'
end

It looks like a great way to link locales to items for me

Why not just use a route like this?

get ':commentable_type/:id/comments', as: :comments
get ':commentable_type/:id/comments/new', as: :new_comment

I have been working on a similar system except that mine is more complex because i have nested resources. I can't get past the first level of nesting with the polymorphic path in the form_for.

For example I have something like this:

Locations with many Building with many Rooms. I want to comment on each with the polymorphic relationship but the system fails when you try to comment on a Building Rails throws an exception looking for 'building_comments_path' but the routing table generates 'location_building_comments_path'.

I am at a loss to generate the polymorphic path on the fly. I could manually generate the routes or the path in my controllers but I figure there has to be a better solution.

Nice episode - had a quick question though. If I wanted to add comments to the index pages eg Articles#index (rather than Articles#show) how would I set up the instance variables @commentable, @comments, @comment?

Hey guys, how can I get the articles using comment object?

Somethin like these:

comment = Comment.all.first
comment.article

supposing the first comment has a comentable_type as article.

Rayan,

Regarding to create a new comment on each model,

link_to "New Comment", [:new, @commentable, :comment]

I have "news" model and "task" model which is nested under "project" model routing

  resources :news
  resources :projects do
    resources :tasks do
      resources :comments
    end
  end

Is there a way to accomplish it so I don't have two links?

Typo in the /app/controllers/comments_controller.rb code block in 'Read the episode'

@commentable = resource.singularize.classify.contantize.find(1)

... missing the 's' in constantize

Having difficulty creating delete and edit links for the polymorphic object.

Anyone know of a good resource on that?

How would you destroy a comment?
I get:

NoMethodError in CommentsController#destroy
undefined methodcomments' for #Comment:0x000000030aae58`

comments controller:

 def destroy
    @comment = @commentable.comments.find(params[:id])
    @comment.destroy

There is an error in the ASCII version

def load_commentable
resource, id = request.path.split('/')[1,2]
@commentable = resource.singularize.classify.contantize.find(1)
end

should be:

def load_commentable
resource, id = request.path.split('/')[1,2]
@commentable = resource.singularize.classify.constantize.find(id)
end

change contantize to constantize and put id between parenthesis.

My understanding is that calling singularize before classify is redundant and can lead to problems with singular forms that end with "s". See http://apidock.com/rails/ActiveSupport/Inflector/classify and http://apidock.com/rails/ActiveSupport/Inflector/camelize. To be safe, classify should only be called on a plural form. The camelize method can be called on either and will return a result with the same grammatical number (i.e. singular or plural). Of note, the source for classify (at least in 3.2.8) is:

camelize(singularize(table_name.to_s.sub(/.*\./, '')))

Repeated calls to singularize are not necessarily safe:

"businesses".singularize # => "business"
"business".singularize   # => "busines"
"busines".singularize    # => "busine"
"busine".singularize     # => "busine"

I have small problem.

I'm using polymorphic on my Review model as reviewable. Where users can review other users and posts.

How will I make it possible to have an owner for each review. BY owner i mean a associated user. Because owner User and the reviews on User are crashing. All good method i can follow or ideas ?

Hello I learned a lot from this episode about Polymorphic association, which always seemed a bit mysterious!

I think what the episode is really missing, is an explanation about polymorphic_url or polymorphic_path in order to recognize which model you wish to redirect. It is extremely helpful to understand this method especially when nesting multiple polymorphic models (see documentation). For example

def update
     @comment = Comment.find(params[:id])
     @comment.update_attributes(params[:comment])
     response_to do |format|
         format.html do
               if @comment.errors.present?
                     render :edit
               else
                     redirect_to polymorphic_path(@commentable), :notice => "Yay updated!"
                     # or
                     # redirect_to polymorphic_path([@commentable, @comment]), :notice => "Yay i can update and see further down the path!"
               end
         end
     end
end

sorry if messed up the comment markup ^^

Thank you Ryan for the wonderful videos. I have been watching your masterpieces for a few years. I love your Pro and Revised versions, Keep it up!

My next polymorphic task is to add this exact function to a small app I have. I thought I was starting easier to just have an address field shared across models but there's not much out there to help with that, especially for Rails 4. I have put together how I got an address polymorphic association working in my app at my blog if anyone needs that help.

http://hamcois.com/articles/6

I have a problem with nested forms and a polymorphic model. Using comments as an example, how does one add multiple has_many :through for the comments needed.
Example:

has_many :comments, :through => :images
has_many :comments, :through => :articles

The second has_many overwrites the first one and :through only allows one reference.

Any tips appreciated.

How do I test this?

I tried:

# RSpec + Shoulda
it {should have_many(:comments).as(:commentable) }

But it doesn't work.

Hey guys,

So in an attempt to get better at Ruby on Rails, I think it would be good for me to build the applications while watching the screen cast. I know Ryan has given us the before and after applications, but when i tried to start the server it showed just the basic information, meaning there was no data (texts or pictures). I am wondering if that is due to something i am doing or is it just not provided.

Thank you

Still a great episode.

If you're using a mountable engine, here's the best way to get the commentable working:

In your routes.rb:

resources :questions do
        resources :comments, :defaults => { :commentable => 'question' }
end

resources :answers do
        resources :comments, :defaults => { :commentable => 'answer' }
end

In your comments_controller.rb:

module MyEngine
  class CommentsController < ApplicationController
          before_action :load_commentable

  private

          def load_commentable
            @commentable = MyEngine::const_get(params[:commentable].classify).find(commentable_id)
          end

          def commentable_id
            params[(params[:commentable].singularize + "_id").to_sym]
          end

  end
end

For mountable engines, you HAVE to do MyEngine:: when constantizing the object. Won't work without it.

Extremly useful episode Thanks Ryan.

I'm currently using disqus.com for comments but I'm having SSO issues so I'm thinking of writing from scratch... Is latency an problem as the number of comments increase? Also, how can you get the comments to chain together? Thanks

This episode has been updated to Rails 5 as a blog post Polymorphic Association in Rails 5