How to only display the product_variants for their parent product using simple_form?

Question

The problem is that I want only the product_variants associated with the product_id displayed on this show page drop down form. Instead, in the dropdown I see all of the product_variants that have been entered in the db -- not just the ones specific to the product.

How would you go about specifying that this simple_form product_variants should only be for @product?

in product show page

<%= simple_form_for [@product_variant, @order_item] do |f| %>
  <%= f.input :quantity %>
  <%= f.association :product_variant, label_method: :name_of_method %>

  <%= f.button :submit, "Add to cart" %>
<% end %>

Currently, the user is able to select a product_variant on each product's show page and succesfully add it to the cart.

Here are my models:

OrderItem

  belongs_to :order, optional: true
  belongs_to :cart, optional: true
  belongs_to :product_variant

  accepts_nested_attributes_for :product_variant

ProductVariant

  belongs_to :product
  has_many :order_items

  validates :item, presence: true


  def name_of_method
  "#{item} / #{color} / #{size} / $#{price_in_dollars}"
  end

  def price_in_dollars
   price / 100
 end

Product

  has_many :product_variants, :dependent => :destroy

  has_many :order_items

Cart

  has_many :order_items

& here are my routes

  # we want to see multiple products
  resources :products

    resources :product_variants do
      resources :order_items
    end


  resources :selected_variants

  # we want a user to only have one cart
  resource :cart

   # we want our users to order multiple times
  resources :orders

  root "products#index"

here is my products controller where code for product_variant display is located in def show:

class ProductsController < ApplicationController

  def index
    @products = Product.all
  end

  def new
    @product = Product.new
  end

  def create
    @product = Product.new(product_params)
    @product.save
  end

  def show

    @product = Product.friendly.find(params[:id])
    @product_variant = ProductVariant.find_by(params[:product_variant_id])

    # if the product is already in the cart
 @order_item = @current_cart.order_items.find_by(product_variant: @product_variant)

   if @order_item.nil?
     # if it doesn't exist in the cart
     @order_item = @current_cart.order_items.new(product_variant: @product_variant, quantity: 1)
   end

  end

  def edit
  end

  def update
    render "edit"
  end

  def destroy
  end



  def product_params
    Params.require(:product).permit(:title, :description, :price, :cover_image, :image_1, :image_2, :image_3 )
  end



end

and here is my schema for relevant tables:

  create_table "product_variants", force: :cascade do |t|
    t.string "item"
    t.string "size"
    t.string "color"
    t.integer "price"
    t.integer "product_id", null: false
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.index ["product_id"], name: "index_product_variants_on_product_id"
  end

  create_table "products", force: :cascade do |t|
    t.string "title"
    t.string "cover_image"
    t.text "description"
    t.integer "price"
    t.string "image_1"
    t.string "image_2"
    t.string "image_3"
    t.boolean "is_sold_out", default: false
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.string "slug"
    t.index ["slug"], name: "index_products_on_slug", unique: true
  end

  create_table "order_items", force: :cascade do |t|
    t.integer "cart_id"
    t.integer "product_id"
    t.integer "product_variant_id"
    t.integer "order_id"
    t.integer "quantity"
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.index ["cart_id"], name: "index_order_items_on_cart_id"
    t.index ["order_id"], name: "index_order_items_on_order_id"
    t.index ["product_id"], name: "index_order_items_on_product_id"
    t.index ["product_variant_id"], name: "index_order_items_on_product_variant_id"
  end

I'll be happy to provide any other code that you may think to be relevant to solve this.

Aaron