Is polymorphic to nested polymorphic models possible?

Post your log file entry where the error occurs so we can see the params that are being passed into the controller and the actual error stack trace. It's very difficult to solve other peoples coding issues with only half the information

ok, you have

ActiveModel::MassAssignmentSecurity::Error (Can't mass-assign protected attributes: link):
  app/controllers/comments_controller.rb:19:in `create'

This tells you that error is occurring on line number 19 in the comments_controller in the create action.

From your post on stack overflow, line 19 is

@comment = @commentable.comments.build(params[:comment])

From the stack trace you posted above the comment
the releant params that are being passed in to your controller are

"comment"=>{"content"=>"comment 1", "link"=>{"link_url"=>"link 1"}}, "commit"=>"Add comment", "forum_post_id"=>"16"}

From this, it's pretty clear that

"link"=>{

is the offensive line.
you have

class Comment < ActiveRecord::Base
  attr_accessible :commentable_id, :commentable_type, :content,:links_attributes, :link_url

  belongs_to :commentable, :polymorphic => true

  has_many :links, :as => :linkable

  accepts_nested_attributes_for :links, :allow_destroy => true  #, :reject_if => lambda { |t| t[:link].nil?}
end

,:links_attributes would accept a param called links. that are nested inside the comments params but your param is called link not links
You have 2 solutions.
Arrange for the param to be called links or arrange for attr_accessible to include link_attributes, which you don't want.
It's a pretty safe bet that what you want to do is to chage your form to post back links rather than link in your params which would require a change to the f.fields_for declaration in your form

  <%= f.fields_for [@linkable, Link.new]  do |link| %>
   <%= render :partial => 'links/link', :locals => { :f => link } %>
  <% end%>

I'm not comfortable with your approach on this and I could have this wrong but you could try

  <%= f.fields_for :links  do |link| %>
   <%= render :partial => 'links/link', :locals => { :f => link } %>
  <% end%>

and build your link on to the comment model in the new action

Last edited by jamesw (2012-10-11 11:55:50)

When the form is posted back to the server The RESTful routes will automatically determine which action is called. Rails standards dictate that the form uses the create action for a new comment record and update action for an existing comment record.
It is standard practice to use a new or an edit action to render the form in the first place. There are exceptions to this and you are obviously frtee to set up whatever routes and actions you want.

I just assumed you had just not included the full CRUD actions for the comments controller in your post. If they are missing then perhaps something went wrong when you scaffolded the comments model?

I suggest that you place that build in whatever action is used to render the form in the first place.

What action is used to render the form?

The problem will lie in either the links not being built, i.e the wrong action being called to render the form, or the build statement is in the wrong action. Amounts to the same thing really.
or
the fields_for delaration is not correct.
Use your log file to check which action is being called that renders the form containing the partial and make sure you use that action to build the links.

Check the fields_for dclaration. I did say that the code I gave you might not be correct as I wasn't comfortable witth your original approach.
The fields_for declaration needs to be able to find the links you have available and I possibly steered you in the wrong direction here. Sorry. You can use the comment object to find all the links

<%= fields_for @comment.links # etc...  %> should do the trick

Read more here http://apidock.com/rails/ActionView/Hel … fields_for