parent item mutations

pkg/cart/mutation_add_item_test.go

@@ -38,3 +38,33 @@ func TestAddItem_MergesByItemIdNotSku(t *testing.T) {
 		t.Fatalf("items = %d, want 2", len(g.Items))
 	}
 }
+
+// Re-adding the same parent (same ItemId + StoreId, no ParentId on the
+// re-add itself) merges the quantity, and the proportional cascade resizes
+// every direct child to match the new parent qty so a "+1 drill" leaves
+// the bundle internally consistent.
+func TestAddItem_MergeCascadesToChildren(t *testing.T) {
+	reg := NewCartMultationRegistry(NewCartMutationContext(nil))
+	g := NewCartGrain(1, time.Now())
+	ctx := context.Background()
+
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 100, Sku: "P", Quantity: 1, Price: 1000}); err != nil {
+		t.Fatalf("add parent: %v", err)
+	}
+	parentLine := g.Items[0].Id
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 101, Sku: "C1", Quantity: 1, Price: 100, ParentId: &parentLine}); err != nil {
+		t.Fatalf("add child: %v", err)
+	}
+
+	// Re-add the same parent — same ItemId, no ParentId on the re-add itself.
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 100, Sku: "P", Quantity: 1, Price: 1000}); err != nil {
+		t.Fatalf("re-add parent: %v", err)
+	}
+
+	// Parent qty 1 -> 2; child qty 1 -> 2 via cascade.
+	for _, it := range g.Items {
+		if it.Quantity != 2 {
+			t.Errorf("line %s qty = %d, want 2 (merge cascade)", it.Sku, it.Quantity)
+		}
+	}
+}