Add Quantifier#positive_stock method

When the quantifier is initialized with a stock location or its ID,
the new method returns the positive amount of stock on hand or zero,
if the stock for the variant is negative. Otherwise, it returns nil.

core/app/models/spree/stock/quantifier.rb

@@ -43,10 +43,23 @@ def can_supply?(required)
         total_on_hand >= required || backorderable?
       end
 
+      def positive_stock
+        return unless stock_location
+
+        on_hand = stock_location.count_on_hand(variant)
+        on_hand.positive? ? on_hand : 0
+      end
+
       private
 
       attr_reader :variant, :stock_location_or_id
 
+      def stock_location
+        @stock_location ||= stock_location_or_id.is_a?(Spree::StockLocation) ?
+          stock_location_or_id :
+          Spree::StockLocation.find_by(id: stock_location_or_id)
+      end
+
       def get_stock_items
         variant.stock_items.select do |stock_item|
           if stock_location_or_id

core/spec/models/spree/stock/quantifier_spec.rb

@@ -12,6 +12,25 @@ module Stock
           expect(subject.can_supply?(100_001)).to eq true
         end
       end
+
+      shared_examples_for "returns the positive stock on hand" do
+        context "when the stock location has no stock for the variant" do
+          it { is_expected.to be_zero }
+        end
+
+        context "when the stock location has negative stock for the variant" do
+          before { stock_item.set_count_on_hand(-1) }
+
+          it { is_expected.to be_zero }
+        end
+
+        context "when the stock location has positive stock for the variant" do
+          before { stock_item.set_count_on_hand(10) }
+
+          it { is_expected.to eq(10) }
+        end
+      end
+
       let(:target_stock_location) { nil }
       let!(:stock_location) { create :stock_location_with_items }
       let!(:stock_item) { stock_location.stock_items.order(:id).first }
@@ -107,6 +126,35 @@ module Stock
           expect(subject.can_supply?(6)).to eq false
         end
       end
+
+      describe "#positive_stock" do
+        let(:variant) { create(:variant) }
+        let(:stock_location) { create(:stock_location) }
+        let(:stock_item) { stock_location.set_up_stock_item(variant) }
+        let(:instance) { described_class.new(variant, stock_location_or_id) }
+
+        subject { instance.positive_stock }
+
+        context "when stock location is not present" do
+          let(:stock_location_or_id) { nil }
+
+          it { is_expected.to be_nil }
+        end
+
+        context "when stock_location_id is present" do
+          context "when stock_location_id is a stock location" do
+            let(:stock_location_or_id) { stock_location }
+
+            it_behaves_like "returns the positive stock on hand"
+          end
+
+          context "when stock_location_id is a stock location id" do
+            let(:stock_location_or_id) { stock_location.id }
+
+            it_behaves_like "returns the positive stock on hand"
+          end
+        end
+      end
     end
   end
 end