Form validation after ajax request - ajax

Question

I'm using Rails 3.2 and I'm making my first Ajax request.
This is my code:
def create
#post = Post.find(params[:post_id])
#comment = Comment.new(params[:comment])
respond_to do |format|
if #comment.save
format.html {
redirect_to #post
}
format.js
else
format.html {
render 'posts/show'
}
format.js
end
end
end
So, I understand that I return a js template.
So, question 1: Is really a good idea to use this approach instead the standard sending json / receiving json in a $.ajax call?
My create.js.erb:
$(".commentlist").append("<%= escape_javascript(render(:partial => #comment)) %>");
Working perfectly.
But... how I validate the form? (Without ajax works perfectly).
I'm seeing that the js.erb is like the html.erb but with js instead of html (I still see this weird, I think that is easier to use controller.coffee, but I will learn this way too :)).
So question 2: How I validate ? I see that what I get in the ajax call is the entire html and my two action vars (#post and #comment).
Just that, I need to learn the Rails approach to do ajax :)
P.S: My _form.html.erb:
<%= form_for comment, :url => post_comments_path(comment.post), :remote => true do |f| %>
<% if comment.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(comment.errors.count, "error") %> prohibited this comment from being added:</h2>
<ul>
<% comment.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<%= f.hidden_field :post_id %>
<p>
<%= f.label :name %>
<%= f.text_field :name %>
</p>
<p>
<%= f.label :email %>
<%= f.text_field :email %>
</p>
<p>
<%= f.label :url %>
<%= f.text_field :url %>
</p>
<p>
<%= f.label :content %>
<%= f.text_area :content %>
</p>
<p class="no-border">
<%= f.submit "Post", :class => "button" %>
</p>
<% end %>