void value expression when accessing constant from view

I'm working on a Rails app using version 2.3.4. I've got a big
form somewhere that uses nested attributes, but I can't seem to
access the constants from the other class.

Here's basically what I'm doing:

class User < ActiveRecord::Base
   has_one :pref
end

class Pref < ActiveRecord::Base
   FOO = 1
end

And in app/views/user.html.erb:

<% form_for( :user, :url => { :action => go_to, :id => id }, :html => { :multipart => true, :id => "user_form" } ) do |f| %>
   <%- fields_for :email_pref do |p| -%>
     <%= p.radio_button :bar, Pref::FOO %>
     <%= p.label :bar, "Bar", :value => Pref::FOO %>
     ...
   <%- end -%>
   ...
<%- end -%>

Of course there's quite a bit more code than that, but this is where it goes
wrong. My error is:

/app/models/pref.rb:59: void value expression

Below it the code of the view, pointing out an error in the first line that uses Pref::FOO
I checked if it really is Pref::FOO that's the problem by adding <%= Pref::FOO %> before
the radiobutton. Also, User::FOO (and declaring the constant in the User class) works
perfectly fine. So it looks like I can't access a Pref constant from a User view, but that
sounds unreasonably limiting to me. I also tried ::Pref::FOO, which also doesn't work.
I really do want this constant in the Pref class. Putting it in User is not a good
solution for me.

Any idea why this is a problem and how to solve it?

And what does this mysterious "void value expression" mean? Googling it yields very few
results (and most of them are the same).

mcv.

Perhaps in your users_controller action(s), you should create an
instance of the Pref class for the view to work with?

Why should I need an instance? Aren't constants tied to the class
rather than an instance?

mcv.