Rails: fields_for only one object

I've got a model Product in my Rails application, its attributes can be edited, and I want to let user comment every change he makes (a comment can be blank, though). So, Product has_many :comments, it accepts_nested_attributes_for :comments and rejects it if the comment is blank.

Hence, the edit form for Product is a multi-model form. The problems I faced are:

1. Fields_for helper renders text areas for all comments belonged to the product, so the user can edit all previous comments. I need it to render fields for the ne开发者_Python百科w one only.
2. If validation breaks, and there are no comments, fields_for renders nothing. Should I perform @product.comments.build in the view before fields_for statement every time, or there is more elegant way to do it?

Maybe I'm wrong and fields_for isn't suitable in this situation?

Base on Tots answer I just made it a little simplier (Rails 3 compatible):

<%= f.fields_for :comments, @product.comments.build do |comment| %>
    <%= comment.label :comments %><br />
    <%= comment.text_area :content %>
<% end %>

<% f.fields_for(:comments, Product.reflect_on_association(:comments).klass.new) 
    do |builder| %>

    <%= builder.label :comment %>
    <%= builder.text_area :comment, :rows => 3 %>
<% end %>