Showing random Product images from same category as the selected product is in. ROR app

I'm modifying a Rails app I've been working on.

I installed a model images.rb to handle all images for products.

When the user has selected a product the user sees the product at app/views/products/show.html.erb

And at the bottom of the show.html.erb the app gives the user suggestions for similar product in the same category as the product the user is looking at.

Before I added the image.rbmodel to handle the images each product had one picture attached to it and every thing worked, but now I'm getting an error. Below is the code I used to display the random products in the same category:

the show.html.erb

 <div class="row product-teaser">
  <h4 class="text-center teaser-text"> similar products to <%= @product.title %> : </h4>
   <% @products_rand.each do |product| %>
      <div class="col-sm-2 col-xs-3 center-block product-thumbs-product-view" >
            <%= link_to product_path (product) do %>
                <%= image_tag @product.image.url, :size => "100%x100%", class: "img-responsive center-block" %>
            <% end %>  
          <h5 class="text-center"><%= link_to product.title, product, class: "text-center" %></h5>    
      </div>
    <% end %>   
  </div>

And in the products_controller.rb show method I had this variable to fetch the random products from the same category.

class ProductsController < ApplicationController
before_action :set_product, only: [:show, :edit, :update, :destroy]

  def show    
     @products_rand = Product.where(category_id: @product.category_id).order("RANDOM()").limit(6)
  end

private
   # Use callbacks to share common setup or constraints between actions.
  def set_product
    @product = Product.find(params[:id])
  end

  # Never trust parameters from the scary internet, only allow the white list through.
  def product_params
    params.require(:product).permit(:title, :description, :price_usd,  :image, :category_id, :stock_quantity, :label_id, :query, :slug, images_attributes: [:image , :id , :_destroy])
  end
 end

After I added the image.rb I always get this error: undefined method 'url' for "":String when trying to load the show page. The error is appearing in this line: <%= image_tag @product.image.url, :size => "100%x100%", class: "img-responsive center-block" %>

the product.rbmodel

class Product < ActiveRecord::Base
  acts_as_list :scope => [:category, :label]
  belongs_to :category
  belongs_to :label

  has_many :images
  accepts_nested_attributes_for :images
  has_many :product_items, :dependent => :destroy

    validates :title, :description, presence: true
    validates :price_usd, :price_isl, numericality: {greater_than_or_equal_to: 0.01}
    validates :title, uniqueness: true  
 end

the image.rbmodel

class Image < ActiveRecord::Base
  belongs_to :product
    has_attached_file :image, styles: { medium: "500x500#", thumb: "100x100#" }
    validates_attachment_content_type :image, content_type: /\Aimage\/.*\z/
 end

I'm not sure what to do, can someone advise me?