create.js.erb

<% if @post.errors.any? %>
    var errors = '';
  <% @comment.errors.full_messages.each do |error| %>
    errors.append("<%= escape_javascript( error ) %>\n");
  <% end %>
  alert(errors);
<% else %>
    $('#posts').prepend('<%=j render 'chapters/posts/post', {:post => @post} %>')
<% end %>