updating part2, far from finished

app/models/product.rb

@@ -21,15 +21,25 @@ class Product
 
   def product_category_ids=(ids)
     @product_category_ids = ids.select(&:present?)
+    is_dirty
   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
+    @product_category_ids ||= []
+    existing_product_categories = product_categories
+
+    # do nothing if nothing has changed
+    return if @product_category_ids == existing_product_categories.map(&:id)
+
+    # clear removed product categories
+    existing_product_categories.reject{|pc| @product_category_ids.include?(pc.id) }.each{|pc| pc.remove_product(self) }
+
+
+    # Add product to newly added product categories
+    database.load(@product_category_ids - existing_product_categories.map(&:id)).each do |product_category|
+      product_category.add_product(self)
     end
   end
 

app/models/section.rb

@@ -130,6 +130,7 @@ class Section
     end
     return saves.all?
   end
+
   def arrange_tables_in_columns_of(n)
     return unless n.present?
     n = n.to_i