Rails 3 association error: undefined method - ruby-on-rails

Question

I've been puzzling over this for quite some time now and can't figure it out.
I've got 2 models:
class Vehicle < ActiveRecord::Base
attr_accessible :year, :capacity,
:size, :body, :model_id, :maker_id, :parameters_attributes
validates :year, numericality: { greater_than: 1900 }
validates :year, :capacity, :size, :body, presence: true
belongs_to :model
belongs_to :maker
has_many :parameters
accepts_nested_attributes_for :parameters
end
and
class Parameter < ActiveRecord::Base
attr_accessible :tag, :value
validates :tag, :value, presence: true
belongs_to :vehicle
end
in new vehicle view i've got:
= form_for [:admin, #vehicle], html: { multipart: true } do |f|
=# some other stuff in between
= f.text_field :value, size: 4
I get this error
undefined method `value'
Just can't seem to get it working. Help, anyone?
EDIT
routes.rb
resources :vehicles
resources :parameters
resources :makers do
resources :models
end

Answer 1

If you are using nested form, you should have something like
f.fields_for :parameters do |parameter|
and than:
parameter.text_field :value, size: 4
Also, remember to create the some parameters in the controller, for example:
def new
#vehicle = Vehicle.new
2.times { #vehicle.parameters.build } #it will create 2 parameters
...
end

Answer 2

f refers to #vehicle, it seems only Parameter bears this field. That's why it fails.
Sidenotes:
In Vehicle you have accepts_nested_attributes_for :parameters but you don't have parameters_attributes in the attr_accessible, can't be good.
If you want to call the relationship in the form consider using fields_for

Answer 3

Ok, I've made a mess of things.
Firstly I've been trying to
def new
#vehicle = #vehicle.parameters.build
end
hence the error undefined method. After a while I got to the correct syntax, which is the one gabrielhilal added after a while.
def new
#vehicle = Vehicle.new
#vehicle.parameters.build
end
No matter ;) Still had problems, because after clicking "create" he wouldn't add records in the database. Turned out that I've set the validates presence: true for tag, but didn't assign any value to it. After fixing that, it worked like a charm. Thanks a lot for all the help.
On to the next puzzle.