Models/tables with relationships and forms

Hi, I’m just starting out writing a RoR application, and I have a
couple of questions…

Lets say I have a model
class Questionnaire < ActiveRecord::Base
has_many :categories
end

I’ve understood that one can reference a variable from the model/table
in a form using e.g.
<%= text_field ‘questionnaire’, ‘title’ %>

But what if I want to do the same with the related model Category?
Using
<%= text_field(‘category’, ‘caption’) %>
does not give any information about which questionnaire the category
belongs to. Are there any neat tricks to do this within the framework,
lets say something like
<%= text_field(‘questionnaire_category_123’, ‘caption’) %>
or must I write some custom code to do this?

And what about validations: using validates_* now only reports that
something is wrong within Categories when i try to do
“some_questionnaire.save”, but does not give me any specific
information about what failed to validate. How can I get that sort of
information?

Any help is greatly appreciated!

Best,
Arnfinn