resources :posts do
    resources :comments
  end

<p><%= @post.title %></p>

    <h2>Comments:</h2>
    <div id='com'>
      <%= render @post.comments %>
    </div>

<h2>Add a comment:</h2>
<%= render "comments/form" %>

<p>
  <strong>Commenter:</strong>
  <%= comment.commenter %>
</p>
<p>
  <strong>Comment:</strong>
  <%= comment.body %>
</p>
<%= link_to "Del", [comment.post, comment], :confirm => 'Are you sure?', :method => :delete %>
<hr>

<%= form_for([@post, @post.comments.build], remote: true) do |f| %>
.
.
.

class CommentsController < ApplicationController

  def create
    @post = Post.find(params[:post_id])
    @comment = @post.comments.create(params[:comment])

    respond_to do |format|
        format.html { redirect_to post_path(@post) }
        format.js
    end
  end

$('#com').html("<%=j render @post.comments %>");

<%= javascript_include_tag 'application' %>

//= require jquery
//= require jquery_ujs