Create polymorphic resources


I have created a polymorphic association for comments since I need commenting functionality on several models. The issue is that when I create the comment in my Commets#create action, I really don’t know what type the parent is of.

So, let’s say I have Foo, Bar and Comment. Comment have this:

belongs_to :commentable, polymorphic: true

And Foo and Bar both have:

has_many :comments, as: :commentable

In my routes.rb I nest the comments:

resources :foo do

resources :comments


resources :bar do

resources :comments


A new comment is posted to /foo/1/comments or /bar/1/comments. When I’m in the Comments#create action, how am I supposed to create the comment through either Foo/Bar?

If it is a comment for a Foo resource, I need to do:

@commentable = Foo.find(params[:foo_id])


But if it’s Bar I need to do:

@commentable = Bar.find(params[:bar_id])


What I do now is to use this method:

def find_commentable

params.each do |name, value|

if name =~ /(.+)_id$/

return $1.classify.constantize.find(value)






@commentable = find_commentable

Which loops over all params for something that ends with _id and then get the class name from that etc… But that feels a bit “hackish” to me :S Another solution would be to add the “commentable_id” and “commentable_type” as hidden fields in my Comment form.

How do you handle this issue? Are there better ways than the way above?



I usually do it like you have already written it, but if you feel uncomfortable with it let me make 2 suggestions (those are quick & dirty and not tested)

  1. In your commentcontroller:

@commentable = params[:foo_id] ? Foo.find(params[:foo_id] : Bar.find(params[:bar_id]

this should work with some tweaking (dont know if it works out of the box, as said, didnt test it) and would be best used if only two models have comments.

  1. In your commentcontroller:

Is to work with some kind of before_filter to determine the commentable object and classify it.

However, I would stay with the above pattern posted by you because it makes it a breeze to add a third, …, n model to it.


I think the best way to go is with the hidden fields. whats your problem with those?