Rest is giving me no rest (more weirdness)

I was here last week working out a correct link_to path for a nested resource.
Through the help I received things were resolved with that issue. Now, I’m having a related issue.

For clarification my routes.rb (the rest parts) is here:

map.resources :candidates do |candidate|
candidate.resources :canbackgrounds

Now originally I had the models set up as such:

class Candidate < ActiveRecord::Base
has_many :canbackgrounds

class Canbackground < ActiveRecord::Base
belongs_to :candidate

However , part of fixing my problem last week was changing the Candidate model to:
class Candidate < ActiveRecord::Base
has_one :canbackground

All was good until I went to candidates/id/canbackgrounds/new
After inputing the data, my controller code (canbackgrounds_controller.rb) threw a no-method error on the create action:
def create
@canbackground = Canbackground.new(params[:canbackground])
@candidate_id = params[:candidate_id]

if (@candidate.canbackgrounds << @canbackground) <-----This was the line, with no method ‘canbackgrounds’

  redirect_to candidate_url(@candidate)

else
render :action => :new
end
end

I thought maybe it needed to be singular but then << threw a no method error.
I went back to the model and changed the Candidate to has_many :canbackgrounds and that fixed the problem.

However if I leave it like this then I’m back to the same problem as last week.
So , one I’m open to hearing if anyone has any ideas about what’s going on , and could the create problem be related
to the fact that a canbackground record already existed when I was attempting to insert another one ?

TIA
Stuart