Deal with edge cases where an adjustment is left referencing a deleted item

app/models/spree/adjustment.rb

@@ -100,6 +100,11 @@ module Spree
     def update_adjustment!(calculable = nil, force: false)
       return amount if immutable? && !force
 
+      if calculable.nil? && adjustable.nil?
+        self.delete
+        return 0.0
+      end
+
       if originator.present?
         amount = originator.compute_amount(calculable || adjustable)
         update_columns(

spec/models/spree/adjustment_spec.rb

@@ -64,6 +64,23 @@ module Spree
         expect(adjustment).not_to receive(:update_columns)
         adjustment.update_adjustment!
       end
+
+      context "where the adjustable has been deleted" do
+        let(:line_item) { create(:line_item, price: 10) }
+        let!(:adjustment) { create(:adjustment, adjustable: line_item) }
+
+        it "returns zero" do
+          line_item.delete
+          expect(adjustment.reload.update_adjustment!).to eq 0.0
+        end
+
+        it "removes orphaned adjustments" do
+          expect {
+            line_item.delete
+            adjustment.reload.update_adjustment!
+          }.to change{ Spree::Adjustment.count }.by -1
+        end
+      end
     end
 
     context "adjustment state" do