No Method Error for "comments", can't find and correct the nil value to make the comment post - ruby-on-rails-3

Question

I'm relatively new to rails and am trying to pull off my first polymorphic association with comments.
I am running rails 3.2.3
Edit - When I try to post a comment, my log is returning this error:
Started POST "/comments" for 127.0.0.1 at 2012-05-20 13:17:38 -0700
Processing by CommentsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"SOLcF71+WpfNLtpBFpz2qOZVaqcVCHL2AVZWwM2w0C4=", "comment"=>{"text"=>"Test this comment"}, "commit"=>"Create Comment"}
User Load (0.3ms) SELECT "users".* FROM "users" WHERE "users"."id" = 101 LIMIT 1
Completed 500 Internal Server Error in 126ms
NoMethodError (undefined method `Comment' for nil:NilClass):
app/controllers/comments_controller.rb:13:in `create'
I have tried out many different solutions offered on SO and elsewhere, including the answer from Jordan below, due, I'm sure, to my own inexperience, but have been unable to resolve the error.
The trace calls out line 13 in the Comments Controller and I commented after that line below to mark the error:
class CommentsController < ApplicationController
def index
#commentable = find_commentable
#comments = #commentable.comments
end
def new
#post = Post.find(params[:post_id])
end
def create
#commentable = find_commentable
#comment = #commentable.comments.build(params[:comment]) #<<<<LINE 13
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
end
Posts Controller:
def show
#post = Post.find(params[:id])
respond_to do |format|
format.html # show.html.erb
format.json { render :json => #post }
end
end
Comment template (in post show)
<ul id="comments">
<% if #comments %>
<h2>Comments</h2>
<% #comments.each do |comment| %>
<li><%= comment.text %></li>
<% end %>
<% else %>
<h2>Comment:</h2>
<% end %>
</ul>
<%= simple_form_for [#commentable,Comment.new], :html => { :class => 'form-horizontal', :multipart => true } do |f| %>
<fieldset>
<%= f.input :text %>
Upload Photo <%= f.file_field :photo %>
</fieldset>
<div class="form-actions">
<%= f.submit nil, :class => 'btn btn-primary' %>
</div>
<% end %>
Post show:
<p id="notice"><%= notice %></p>
<div class="row">
<div class="span2 offset1">
<%= image_tag #post.photo.url(:show) %>
</div>
<div class="span5">
<h1><%= #post.title %></h1>
<p><%= #post.index_text.html_safe %></p>
<p><%= #post.show_text.html_safe %></p>
<%= render "comments/comment" %>
<%= render "comments/form" %>
<% if can? :update, #course %>
<%= link_to 'Edit Post', edit_post_path(#post), :class => 'btn btn-mini' %>
<%= link_to 'Delete Post', #post,
confirm: 'Are you sure?',
method: :delete,
:class => 'btn btn-mini' %>
<%= link_to 'New Post', new_post_path, :class => 'btn btn-mini' %>
<% end %>
</div>
<nav class="span2 offset1">
<ul class="well">
<li>Category 1</li>
<li>Category 2</li>
</ul>
</nav>
</div>
<div class="row offset2">
<%= link_to 'Back to Posts', posts_path, :class => 'btn btn-mini' %>
</div>
Routes:
resources :posts, :has_many => :comments
resources :comments
It is probably something obvious that someone with more experience can resolve. Let me know if anything comes to mind. Brian

Answer 1

The problem is that #commentable is nil, which means that CommentsController#find_commentable is returning nil. I think your regular expression is sound, so that means one of two things is happening in find_commentable:
There aren't any keys in params that match your regex.
Your regex is matching but there aren't any records in the resulting table with the id in value.
Debug this as usual by inspecting params and the records in your database to make sure they look like you expect them to look.

Answer 2

The problem is your find_commentable method.
Here are the params passed to your CommentsController#create:
Started POST "/comments" for 127.0.0.1 at 2012-05-20 13:17:38 -0700
Processing by CommentsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"SOLcF71+WpfNLtpBFpz2qOZVaqcVCHL2AVZWwM2w0C4=", "comment"=>{"text"=>"Test this comment"}, "commit"=>"Create Comment"}
Here is your CommentsController#create:
def create
#commentable = find_commentable
#comment = #commentable.comments.build(params[:comment]) #<<<<LINE 13
def find_commentable
params.each do |name, value|
if name =~ /(.+)_id$/
return $1.classify.constantize.find(value)
end
end
nil
end
As you can see, find_commentable expects a param like xx_id (for example, comments_id) which it uses to search for an appropriate class (in case of comments_id, it will be Comment), otherwise it returns nil. Refer classify and constantize here.
Your params do not contain any such param. So, you always get a nil object.
Your find_commentable needs some rework. I think in case of nested_fields, it should be an expression like
/(.+)_attributes$/
instead of
/(.+)_id$/.
And you need to have
:accepts_nested_attributes_for :commentable
in your Comment model class.

Answer 3

I tried both of the above answers, but the problem continued.
I ended up consulting with a friend who suggested the following solution, which I like because it's more elegant than my original attempt and easier to read (for later, when I or someone else need to return to the code):
def find_commentable
if params[:post_id]
Post.find(params[:post_id])
#elsif params[:other_id]
# Other.find(params[:other_id])
else
# error out?
end
end
The commented out section will refer to other associations once I get them up and running.