[Rails] has_many :through

Hi

I have a problem with has_many :trough association. I have 2 tables, Product and Category, and when I create new Product, I should make checkboxes to choose in what categories the product . I've made model category_product where I store the association from the Category and Product tables. Here is some of the code.

class Category < ActiveRecord::Base

  has_many :category_products, dependent: :destroy
  has_many :products, :through => :category_products

  accepts_nested_attributes_for :category_products




class CategoryProduct < ActiveRecord::Base

  belongs_to :category
  belongs_to :product
  accepts_nested_attributes_for :category




class Product < ActiveRecord::Base

  has_many :category_products, dependent: :destroy
  has_many :categories, :through => :category_products

  accepts_nested_attributes_for :category_products





class ProductsController < ApplicationController

def new
    @product = Product.new
  end

  def create

    @product = Product.new(product_params)

    if @product.save

      items_array = category_params
      items_array.each do |key, value|

          if value != ""
            @category_product = CategoryProduct.new(:category => value, :product => @product.id)
            @category_product.save
          end

      #category_products.create!(:category => arrayitem.id, :product => @product.id)
      end

      #@categories = Category.find(category_params)


      # @categories.each do |category|
        #category_products.create!(:category => category.id, :product => @product.id)