Add Variant stock API of Spree 1

app/models/spree/variant_decorator.rb

@@ -1,5 +1,6 @@
 require 'open_food_network/enterprise_fee_calculator'
 require 'open_food_network/variant_and_line_item_naming'
+require 'open_food_network/variant_stock'
 require 'open_food_network/products_cache'
 
 Spree::Variant.class_eval do
@@ -9,6 +10,7 @@ Spree::Variant.class_eval do
   # removing the Spree method to prevent error.
   remove_method :options_text if instance_methods(false).include? :options_text
   include OpenFoodNetwork::VariantAndLineItemNaming
+  include OpenFoodNetwork::VariantStock
 
   has_many :exchange_variants
   has_many :exchanges, through: :exchange_variants

lib/open_food_network/variant_stock.rb

@@ -0,0 +1,80 @@
+require 'active_support/concern'
+
+# These methods were available in Spree 1, but were removed in Spree 2.
+# We would still like to use them. Therefore we use only a single stock location
+# (default stock location) and use it to track the `count_on_hand` value that
+# was previously a database column on variants.
+#
+# We may decide to deprecate these methods after we designed the Network feature.
+module OpenFoodNetwork
+  module VariantStock
+    extend ActiveSupport::Concern
+
+    included do
+      after_save :save_stock
+    end
+
+    def on_hand
+      if on_demand
+        Float::INFINITY
+      else
+        total_on_hand
+      end
+    end
+
+    def count_on_hand
+      total_on_hand
+    end
+
+    def on_hand=(new_level)
+      error = 'Cannot set on_hand value when Spree::Config[:track_inventory_levels] is false'
+      raise error unless Spree::Config.track_inventory_levels
+
+      self.count_on_hand = new_level
+    end
+
+    def count_on_hand=(new_level)
+      raise_error_if_no_stock_item_available
+      overwrite_stock_levels new_level
+    end
+
+    def on_demand
+      stock_items.any?(&:backorderable?)
+    end
+
+    def on_demand=(new_value)
+      raise_error_if_no_stock_item_available
+
+      # There should be only one at the default stock location.
+      stock_items.each do |item|
+        item.backorderable = new_value
+      end
+    end
+
+    private
+
+    def save_stock
+      stock_items.each(&:save)
+    end
+
+    def raise_error_if_no_stock_item_available
+      message = 'You need to save the variant to create a stock item before you can set stock levels.'
+      raise message if stock_items.empty?
+    end
+
+    # Backwards compatible setting of stock levels in Spree 2.0.
+    # It would be better to use `Spree::StockItem.adjust_count_on_hand` which
+    # takes a value to add to the current stock level and uses proper locking.
+    # But this should work the same as in Spree 1.3.
+    def overwrite_stock_levels(new_level)
+      stock_items.first.send :count_on_hand=, new_level
+
+      # There shouldn't be any other stock items, because we should have only one
+      # stock location. But in case there are, the total should be new_level,
+      # so all others need to be zero.
+      stock_items[1..-1].each do |item|
+        item.send :count_on_hand=, 0
+      end
+    end
+  end
+end

spec/factories.rb

@@ -483,8 +483,19 @@ FactoryBot.modify do
   end
 
   factory :variant do
+    transient do
+      on_demand { false }
+      on_hand { 5 }
+    end
+
     unit_value 1
     unit_description ''
+
+    after(:create) do |variant, evaluator|
+      variant.on_demand = evaluator.on_demand
+      variant.count_on_hand = evaluator.on_hand
+      variant.save
+    end
   end
 
   factory :shipping_method do