Require count on hand in non backorderable StockItem to be positive or zero

Fix setting of count on hand in line item specs

app/models/spree/stock_item.rb

@@ -10,6 +10,7 @@ module Spree
     # rubocop:disable Rails/UniqueValidationWithoutIndex
     validates :variant_id, uniqueness: { scope: :stock_location_id }
     # rubocop:enable Rails/UniqueValidationWithoutIndex
+    validates :count_on_hand, numericality: { greater_than_or_equal_to: 0, unless: :backorderable? }
 
     attr_accessible :count_on_hand, :variant, :stock_location, :backorderable, :variant_id
 

spec/models/spree/line_item_spec.rb

@@ -97,7 +97,7 @@ module Spree
       end
 
       it "caps at zero when stock is negative" do
-        v.update! on_hand: -2
+        v.__send__(:stock_item).update_column(:count_on_hand, -2)
         li.cap_quantity_at_stock!
         expect(li.reload.quantity).to eq 0
       end
@@ -123,7 +123,7 @@ module Spree
           before { vo.update(count_on_hand: -3) }
 
           it "caps at zero" do
-            v.update(on_hand: -2)
+            v.__send__(:stock_item).update_column(:count_on_hand, -2)
             li.cap_quantity_at_stock!
             expect(li.reload.quantity).to eq 0
           end

spec/models/spree/stock_item_spec.rb

@@ -7,6 +7,33 @@ RSpec.describe Spree::StockItem do
 
   subject { stock_location.stock_items.order(:id).first }
 
+  describe "validation" do
+    let(:stock_item) { stock_location.stock_items.first }
+
+    it "requires count_on_hand to be positive if not backorderable" do
+      stock_item.backorderable = false
+
+      stock_item.__send__(:count_on_hand=, 1)
+      expect(stock_item.valid?).to eq(true)
+
+      stock_item.__send__(:count_on_hand=, 0)
+      expect(stock_item.valid?).to eq(true)
+
+      stock_item.__send__(:count_on_hand=, -1)
+      expect(stock_item.valid?).to eq(false)
+    end
+
+    it "allows count_on_hand to be negative if backorderable" do
+      stock_item.backorderable = true
+
+      stock_item.__send__(:count_on_hand=, 1)
+      expect(stock_item.valid?).to eq(true)
+
+      stock_item.__send__(:count_on_hand=, -1)
+      expect(stock_item.valid?).to eq(true)
+    end
+  end
+
   it 'maintains the count on hand for a variant' do
     expect(subject.count_on_hand).to eq 15
   end
@@ -53,7 +80,10 @@ RSpec.describe Spree::StockItem do
       let(:inventory_unit) { double('InventoryUnit') }
       let(:inventory_unit_2) { double('InventoryUnit2') }
 
-      before { subject.adjust_count_on_hand(- (current_on_hand + 2)) }
+      before do
+        allow(subject).to receive(:backorderable?).and_return(true)
+        subject.adjust_count_on_hand(- (current_on_hand + 2))
+      end
 
       it "doesn't process backorders" do
         expect(subject).not_to receive(:backordered_inventory_units)