Rails, How i can create grouped_select? - ruby-on-rails

Question

I'm new in rails and i need a help.
I wanna to do a grouped selection on rails but i dont know how i can do it.
i have 3 db tables cars, car_brands and car_models. When i add new car i need to select car model, how i can do it with gtouped selection.
models/car_brand.rb
class CarBrand < ActiveRecord::Base
has_many :cars
has_many :car_models
mount_uploader :logo, CarBrandImgUploader
end
models/car_model.rb
class CarModel < ActiveRecord::Base
has_many :cars
belongs_to :car_brand
end
cars_controller.rb
class CarsController < ApplicationController
before_action :set_car, only: [:show, :edit, :update, :destroy]
before_action :set_model, only: [:index, :new, :edit]
# GET /cars
# GET /cars.json
def index
#cars = Car.all
end
# GET /cars/1
# GET /cars/1.json
def show
end
# GET /cars/new
def new
#car = Car.new
end
# GET /cars/1/edit
def edit
end
# POST /cars
# POST /cars.json
def create
#car = Car.new(car_params)
respond_to do |format|
if #car.save
if params[:ImagesCars]
params[:ImagesCars]['image'].each do |a|
#ImagesCar = #car.ImagesCars.create!(:image => a, :car_id => #car.id)
end
end
format.html { redirect_to #car, notice: 'Car was successfully created.' }
format.json { render :show, status: :created, location: #car }
else
format.html { render :new }
format.json { render json: #car.errors, status: :unprocessable_entity }
end
end
end
# PATCH/PUT /cars/1
# PATCH/PUT /cars/1.json
def update
respond_to do |format|
if #car.update(car_params)
if params[:ImagesCars]
params[:ImagesCars]['image'].each do |a|
#ImagesCar = #car.ImagesCars.create!(:image => a, :car_id => #car.id)
end
end
format.html { redirect_to #car, notice: 'Car was successfully updated.' }
format.json { render :show, status: :ok, location: #car }
else
format.html { render :edit }
format.json { render json: #car.errors, status: :unprocessable_entity }
end
end
end
# DELETE /cars/1
# DELETE /cars/1.json
def destroy
#car.destroy
respond_to do |format|
format.html { redirect_to cars_url, notice: 'Car was successfully destroyed.' }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
def set_car
#car = Car.find(params[:id])
end
def set_model
#models = CarModel.all
#brands = CarBrand.all
end
# Never trust parameters from the scary internet, only allow the white list through.
def car_params
params.require(:car).permit(:title_en, :keys_en, :description_en, :text_en, :title_fr, :keys_fr, :description_fr, :text_fr, :title_ru, :keys_ru, :description_ru, :text_ru, :title_es, :keys_es, :description_es, :text_es, :model_id, :brand_id, :price_day, :price_week, :p_info, :images_cars => [:id, :car_id, :image])
end
end
cars/new.html.haml
= simple_form_for(#car, html: { role: 'form', multipart: true }) do |f|
= f.input :price_day, as: :integer, input_html: {class: 'form-control', placeholder: 'Price Day ej: 150'}
= f.cktext_area :text_en, as: :text, input_html: { class: 'form-control' }, :label => 'EN Website Content Text EN'
=f.input :model_id, collection: #models, as: :grouped_select, group_method: :brand_id
= f.submit 'Save'
i get this error:
undefined method `map' for nil:NilClass
Please help or explain how i can do the grouped selection.
Thx