Nested Forms, Validations and Error Messages - ruby-on-rails

Question

So after two days of reading up on this topic I feel lost between problems in older versions of rails and some individual stuff that I probably did wrong.
I have a form where a User can create an account. Every user needs to put in his number plate and a car is created if it does not yet exist with that number plate and assigned to the user.
My form shows the validation error messages for the user, but not for the car (it only marks the input fields for the number plates red).
So my questions is: How do I get the error messages to render be shown properly?
I also suspect after all the topics I read, that my user controller might be doing some stuff that I should not do manually.
My form:
<%= form_for(#user, html: { multipart: true }) do |f| %>
<%= render 'shared/error_messages', object: f.object %>
<div class="row">
<div class="col-xs-6 col-xs-offset-3">
<%= render 'shared/error_messages', object: f.object %>
<%= f.label :name %>
<%= f.text_field :name, class: 'form-control' %>
<%= f.label :email %>
<%= f.email_field :email, class: 'form-control' %>
<%= f.label :password %>
<%= f.password_field :password, class: 'form-control' %>
<%= f.label :password_confirmation, "Bestätigung" %>
<%= f.password_field :password_confirmation, class: 'form-control' %>
</div>
</div>
<div class="row">
<div class="col-xs-6 col-xs-offset-3">
<%= f.fields_for :car, #car do |car| %>
<%= car.label :plateprefix, "Nummernschild" %>
<div class="form-inline">
<%= car.text_field :plateprefix %>
<%= car.label :plate, "-" %>
<%= car.text_field :plate %>
</div>
<% end %>
</div>
</div>
user.rb:
class User < ActiveRecord::Base
belongs_to :car, inverse_of: :users
validates :name, presence: true, length: { maximum: 50 }
car.rb:
class Car < ActiveRecord::Base
has_many :users, inverse_of: :car
accepts_nested_attributes_for :users
validates :plateprefix, presence: true
and finally my car controller:
def new
#user = User.new
end
def create
if verify_recaptcha
#user = User.new(user_params)
if Car.exists?(:country => 'D', :plateprefix => params[:user][:car][:plateprefix].upcase, :plate => params[:user][:car][:plate].upcase)
#car = Car.find_by(:country => 'D', :plateprefix => params[:user][:car][:plateprefix].upcase, :plate => params[:user][:car][:plate].upcase)
#user.car_id = #car.id
else
#car = Car.new(:country => 'D', :plateprefix => params[:user][:car][:plateprefix].upcase, :plate => params[:user][:car][:plate].upcase)
#car.country = 'D'
#car.save
#user.car_id = #car.id
end
if #user.save
#user.send_activation_email
flash[:info] = "Please check your email to activate your account."
redirect_to root_url
else
#user = User.new
render 'new'
end
else
#user = User.new
render 'new'
end
end

Answer 1

Ok, so after 2 further days of reading I solved it:
First I moved the accepts_nested_attributes from the car.rb to the user.rb (and from plural to singular)
user.rb
accepts_nested_attributes for :car
but then for some reason in my form I had to change the variable in the fields_for to plural (:cars - some of rails magic is beyond me but I am getting there I think)
Next i also recreated my controller to use the methods provided by ActiveModel/Record and went to do:
if Car.exists?(:country => 'D', :plateprefix => params[:user][:cars][:plateprefix].upcase, :plate => params[:user][:cars][:plate].upcase)
#user.car = Car.find_by(:country => 'D', :plateprefix => params[:user][:cars][:plateprefix].upcase, :plate => params[:user][:cars][:plate].upcase)
else
#user.create_car(:country => 'D', :plateprefix => params[:user][:cars][:plateprefix].upcase, :plate => params[:user][:cars][:plate].upcase)
end
So this cleaned up my code a bit and I finally got my errors to show properly!