## posts / show.html.erb

<h1><%= h @post.title %></h1>
<div class="post">
  <p class="created_at">Posted <%= pretty_datetime(@post.created_at) %></p>
  <p><%= sanitize @post.body %></p>
</div>

<% unless @post.comments.empty? -%>
<div class="comments">
  <h2>Comments</h2>
  <%= render :partial => @post.comments %>
</div>
<% end -%>

<div class="comment_form">
  <h2>Post a comment</h2>
  <%= error_messages_for :comment %>
  <% form_for :comment, :url => post_comments_path(@post.id) do |f| %>
  <div>Name</div>
  <div><%= f.text_field :author %></div>
  <div>Email <span class="small gray">&mdash; optional, shown to public</span></div>
  <div><%= f.text_field :email %></div>
  <div>Comment</div>
  <div><%= f.text_area :body, :size => "30x6" %></div>
  <div><%= submit_tag "Post comment" %></div>
</div>
<% end -%>

## comments_controller.rb

class CommentsController < ApplicationController

  def create
    post_id = params[:post_id]
    @post = Post.find(post_id)
    @comment = Comment.new(params[:comment])
    @comment.post_id = post_id
    if @comment.save
      # Validations passed
      flash[:notice] = "Your comment has been saved, #{@comment.author.titleize}."
      redirect_to post_path(@post.permalink)
    else
      render post_path(@post.permalink)
    end
  end

  def new
    @comment = Comment.new
  end

end

## comment.rb

class Comment < ActiveRecord::Base
  validates_presence_of :name, :comment
  validates_format_of :email, :with => /\A\w+\@\w+\.\w+\Z/, :unless => :email_blank?
  belongs_to :post, :counter_cache => true # comments.post_id

  def email_blank?
    self.email.blank?
  end
end

## Error when submitting blank comment

ActionController::RenderError in CommentsController#create

You called render with invalid options : /posts/nullam-tincidunt
RAILS_ROOT: /Users/kip/svn/doohickey

Application Trace | Framework Trace | Full Trace
/opt/local/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:841:in `render_with_no_layout'
/opt/local/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/layout.rb:262:in `render_without_benchmark'
/opt/local/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in `render'
/opt/local/lib/ruby/1.8/benchmark.rb:293:in `measure'
/opt/local/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in `render'
app/controllers/comments_controller.rb:13:in `create'
Request

Parameters:

{"commit"=>"Post comment",
 "post_id"=>"3",
 "authenticity_token"=>"2a1e1a9c38c2ea506372602d09929a8c34cedd1f",
 "comment"=>{"body"=>"",
 "author"=>"",
 "email"=>""}}

## Now, after changing the controller to this, here are my results:

## New comments_controller

class CommentsController < ApplicationController

  def create
    post_id = params[:post_id]
    @post = Post.find(post_id)
    @comment = Comment.new(params[:comment])
    @comment.post_id = post_id
    if @comment.save
      # Validations passed
      flash[:notice] = "Your comment has been saved, #{@comment.author.titleize}."
      redirect_to :controller => "/posts", :id => @post.permalink, :template => "posts/show"
    else
      render :controller => "/posts", :id => @post.permalink, :template => "posts/show"
    end
  end

  def new
    @comment = Comment.new
  end

end

## posts_controller.rb (commented out line 3)

  def show
    @post = Post.find_by_permalink(params[:id])
    # @comment = Comment.new
  end

## When submitting VALID content for comment:

Name can't be blank
Comment can't be blank

## params.inspect on show.html.erb

{"commit"=>"Post comment", "authenticity_token"=>"2a1e1a9c38c2ea506372602d09929a8c34cedd1f", "action"=>"create", "post_id"=>"3", "controller"=>"comments", "comment"=>{"body"=>"comment", "author"=>"name", "email"=>"me@me.com"}}