Rails - Sep 2012 - curd on nested resources(the comments resource in rails user guide exmaple)

Hi:

I am following the rails 3 user guide,the blog application.

And I want to make the comment resources editable(and both the
create/update form will be displayed in the post/show page),so I try to
make the following modifiation(the core):

CommentController.rb:
class CommentsController < ApplicationController
  def edit
    @post=Post.find(params[:post_id])
    @comments=Comment.all
    @comment = Comment.find(params[:id])
    render "/posts/show"
  end

  def create
    @post = Post.find(params[:post_id])
    @comment = @post.comments.create(params[:comment])
    respond_to do |format|
      if @comment.save
        format.html { redirect_to post_path @post, notice: ''Comment was
successfully saved.'' }
        format.json { head :no_content }
      else
        format.html { render "posts/show" }
        format.json { render json: @comment.errors, status:
:unprocessable_entity }
      end
    end
  end

  def update
    @post=Post.find(params[:post_id])
    @comment = Comment.find(params[:id])
    respond_to do |format|
      if @comment.update_attributes(params[:comment])
        format.html { redirect_to post_path(@post), notice: ''Comment
was
successfully updated.'' }
        format.json { head :no_content }
      else
        format.html { render "posts/show" }
        format.json { render json: @comment.errors, status:
:unprocessable_entity }
      end
    end
  end
end

post/show.erb.html:

    <h2>Comments</h2>
    <% @post.comments.each do |comment| %>
    <tr>
        <td><%= comment.commenter %></td>
        <td><%= comment.body %></td>
        <td><%= link_to ''Edit'',
edit_post_comment_path(@post,comment)
%></td>
        <td><%= link_to ''Destroy'',
post_comment_path(@post,comment),
confirm: ''Are you sure?'', method: :delete %></td>
    </tr>
    <% end %>

<h2>Add a comment:</h2>
<%= form_for([@post,@comment]) do |f| %>
<% if @comment.errors.any? %>
<div id="error_explanation">
    <h2><%= pluralize(@comment.errors.count, "error") %>
prohibited this
comment from being saved:</h2>

    <ul>
        <% @comment.errors.full_messages.each do |msg| %>
        <li>
            <%= msg %>
        </li>
        <% end %>
    </ul>
</div>
<% end %>
<div class="field">
    <%= f.label :commenter %>
    <br />
    <%= f.text_field :commenter %>
</div>
<div class="field">
    <%= f.label :body %>
    <br />
    <%= f.text_area :body %>
</div>
<div class="actions">
    <%= f.submit %>
</div>
<% end %>

routes.rb
resources :posts do
  resources :comments
end

The above codes seems work at first,since I can CURD for posts and the
nested comments.

However when I create a new comment,if there was any errors in the user
input(for example minimum requirement),I will meet the error:

No route matches {:action=>"edit",
:controller=>"comments" ......

When I remove the lines:
        <td><%= link_to ''Edit'',
edit_post_comment_path(@post,comment)
%></td>
        <td><%= link_to ''Destroy'',
post_comment_path(@post,comment),
confirm: ''Are you sure?'', method: :delete %></td>
It will pass, and this error will not occur when I update a comment.

So I wonder why?

BTW,I am afraid I can not express clearly,so I upload the source codes,I
hope someone can do me a favor and check it. Thanks.

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