Move tax_category association from product to variant

app/models/spree/product.rb

@@ -31,7 +31,6 @@ module Spree
     searchable_associations :supplier, :properties, :primary_taxon, :variants
     searchable_scopes :active, :with_properties
 
-    belongs_to :tax_category, class_name: 'Spree::TaxCategory'
     belongs_to :shipping_category, class_name: 'Spree::ShippingCategory'
     belongs_to :supplier, class_name: 'Enterprise', touch: true
     belongs_to :primary_taxon, class_name: 'Spree::Taxon', touch: true
@@ -59,8 +58,6 @@ module Spree
 
     validates :supplier, presence: true
     validates :primary_taxon, presence: true
-    validates :tax_category, presence: true,
-                             if: proc { Spree::Config[:products_require_tax_category] }
 
     validates :variant_unit, presence: true
     validates :unit_value, presence:
@@ -205,14 +202,6 @@ module Spree
       group(column_names.map { |col_name| "#{table_name}.#{col_name}" })
     end
 
-    def tax_category
-      if self[:tax_category_id].nil?
-        TaxCategory.find_by(is_default: true)
-      else
-        TaxCategory.find(self[:tax_category_id])
-      end
-    end
-
     # for adding products which are closely related to existing ones
     def duplicate
       duplicator = Spree::Core::ProductDuplicator.new(self)

app/models/spree/variant.rb

@@ -28,9 +28,10 @@ module Spree
                        supplier_name).join('_or_')}_cont".freeze
 
     belongs_to :product, -> { with_deleted }, touch: true, class_name: 'Spree::Product'
+    belongs_to :tax_category, class_name: 'Spree::TaxCategory'
 
-    delegate_belongs_to :product, :name, :description, :tax_category_id, :shipping_category_id,
-                        :meta_keywords, :tax_category, :shipping_category
+    delegate_belongs_to :product, :name, :description, :shipping_category_id,
+                        :meta_keywords, :shipping_category
 
     has_many :inventory_units, inverse_of: :variant
     has_many :line_items, inverse_of: :variant
@@ -61,8 +62,9 @@ module Spree
     localize_number :price, :weight
 
     validate :check_currency
-    validates :price, numericality: { greater_than_or_equal_to: 0 },
-                      presence: true
+    validates :price, numericality: { greater_than_or_equal_to: 0 }, presence: true
+    validates :tax_category, presence: true,
+                             if: proc { Spree::Config[:products_require_tax_category] }
 
     validates :unit_value, presence: true, if: ->(variant) {
       %w(weight volume).include?(variant.product&.variant_unit)
@@ -166,6 +168,14 @@ module Spree
                                           select("spree_variants.id"))
     end
 
+    def tax_category
+      if self[:tax_category_id].nil?
+        TaxCategory.find_by(is_default: true)
+      else
+        TaxCategory.find(self[:tax_category_id])
+      end
+    end
+
     def price_with_fees(distributor, order_cycle)
       price + fees_for(distributor, order_cycle)
     end