Model Validation with Multiple Models and Good MVC Design

1. Model Validation With Multiple Controllers Involved
I have a Posts controller and model and a Comments controller and
model. Posts has a show method and a corresponding view that shows a
post and its associated comments, which are rendered with a partial
from comments. Another partial from comments is used to allow the user
to add their comment to the page.

Problem: If a user tries to add a new comment but it fails the comment
model validation, how do I get back to the show page from posts and
display the model validation error for the new comment?

#spit out the details of the post
#render the existing comments and let a user make a new one
<%= render :partial => "comments/comment", :collection =>
@post.comments %>
<%= render :partial => "comments/new"%>

#new method unnecessary because the "comments/new" partial never calls
def new
  @comment =

def create
  @comment =[:comment])
    redirect_to :controller => "posts", :action => "show", :id =>
    #we're over in the comments controller - how do we get our data
validation errors from the
    #comments model back over to the show page of posts?
    render :controller => "posts", :action => "show"

2. Good MVC Design
OK so for my little blog app at first I had just one controller, blog,
and just kept adding methods there like add_post, add_comment, etc. It
seemed like maybe that was a bad idea so I decided to use three
controllers instead of one: blog, posts, and comments. Blog would be
light-weight and not have to care about the details of posts or
comments. But then I ran into the problem described above.

What would be the most natural way to structure this kind of thing
with MVC, one blog controller with double-barelled methods like
add_post, add_comment etc. or a bunch of controllers like posts,
comments, etc. that do all the heavy lifting?