index.html.erb

<div class="row">
    <% @categories.each do |category|%>
        <h2><%=category.name%></h2>
        <%= image_tag category.image.thumb.url%>
        <ul>
        <%= category.products.published.each do |product| %>
            <li><%=product.name%></li>
             <li><%=product.price%></li>
             <li><%=product.description%></li>
          <% end %>
      </br>
        </ul>
    <% end %>
</div>

Product.rb

class Product < ApplicationRecord
   scope :published, -> {
     where(:is_published => true)
   }
   
   belongs_to :category
   
   validates_presence_of :name, :price
   mount_uploader :image, ImageUploader
   validates_processing_of :image
   validate :image_size_validation
   
   private
     def image_size_validation
       errors[:image] << "Η εικόνα πρέπει να είναι μικρότερη απο 3MB." if image.size > 3.megabytes
     end
end

product_controller.rb

class ProductsController < ApplicationController
  def index
     @categories = Category.published.order("position")
     @products = Product.where(is_published: true).order("position")
  end
 
  def show
  end
end