many changes

app/models/product.rb

@@ -5,10 +5,26 @@ class Product
   property :code
   property :price, type: Float
 
-  belongs_to :product_category
+  #belongs_to :product_category
+  has_and_belongs_to_many :product_categories, storing_keys: false
   belongs_to :supplier # direct! category is an aid
   has_many :product_orders
 
   validates :supplier_id, presence: true
+  after_save :persist_product_category_ids
+
+  def product_category_ids=(ids)
+    @product_category_ids = ids.select(&:present?)
+  end
+
+private
+  def persist_product_category_ids
+    return unless @product_category_ids.present?
+    database.load(@product_category_ids).each do |product_category|
+      product_category.product_ids ||= []
+      product_category.product_ids |= [id]
+      product_category.save
+    end
+  end
 
 end