mutation fixes

pkg/cart/mutation_add_item_test.go

@@ -68,3 +68,125 @@ func TestAddItem_MergeCascadesToChildren(t *testing.T) {
 		}
 	}
 }
+
+// Item identity for merge purposes includes ParentId: a standalone root
+// line (ParentId == nil) and a child line (ParentId == &someParentLine) that
+// happen to share an ItemId are different roles and MUST stay as distinct
+// lines. Merging would silently turn an accessory into a +N on the root
+// (or vice versa), corrupting the parent-child link.
+func TestAddItem_DoesNotMergeWhenParentIdDiffers(t *testing.T) {
+	reg := NewCartMultationRegistry(NewCartMutationContext(nil))
+	g := NewCartGrain(1, time.Now())
+	ctx := context.Background()
+
+	// Standalone root (no ParentId).
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 100, Sku: "P", Quantity: 1, Price: 1000}); err != nil {
+		t.Fatalf("add standalone: %v", err)
+	}
+	parentLine := g.Items[0].Id
+
+	// Same ItemId+StoreId but with ParentId set — must create a NEW line,
+	// not bump the standalone's qty.
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 100, Sku: "P", Quantity: 1, Price: 1000, ParentId: &parentLine}); err != nil {
+		t.Fatalf("add child-form: %v", err)
+	}
+
+	if len(g.Items) != 2 {
+		t.Fatalf("items = %d, want 2 (different ParentId => distinct lines)", len(g.Items))
+	}
+	// Standalone untouched at qty 1; child-form is its own line at qty 1.
+	for _, it := range g.Items {
+		if it.Quantity != 1 {
+			t.Errorf("line %d (parentId=%v) qty = %d, want 1 (no merge)", it.Id, it.ParentId, it.Quantity)
+		}
+		// One of them has nil ParentId and the other has &parentLine — guard
+		// against an accidental merge ever flipping both to non-nil.
+	}
+	var seenNil, seenSet bool
+	for _, it := range g.Items {
+		if it.ParentId == nil {
+			seenNil = true
+		} else if *it.ParentId == parentLine {
+			seenSet = true
+		}
+	}
+	if !seenNil || !seenSet {
+		t.Errorf("expected one line with nil ParentId and one with &parentLine; got nil=%v set=%v", seenNil, seenSet)
+	}
+}
+
+// Positive control: when BOTH sides have ParentId set to the SAME parent
+// line, the merge proceeds (this guards the new sameParent guard against
+// accidentally rejecting a legitimate "re-add same child of same parent"
+// case — e.g. an idempotent retry from a flaky request).
+func TestAddItem_MergesWhenBothParentIdMatch(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
+
+	// First child: ItemId 200 under parent.
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 200, Sku: "C", Quantity: 1, Price: 100, ParentId: &parentLine}); err != nil {
+		t.Fatalf("add child: %v", err)
+	}
+	// Re-add the SAME child (same ItemId+StoreId+ParentId) — must merge into
+	// qty 2, not create a second accessory line.
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 200, Sku: "C", Quantity: 1, Price: 100, ParentId: &parentLine}); err != nil {
+		t.Fatalf("re-add child: %v", err)
+	}
+
+	if len(g.Items) != 2 {
+		t.Fatalf("items = %d, want 2 (parent + merged child)", len(g.Items))
+	}
+	for _, it := range g.Items {
+		switch it.Id {
+		case parentLine:
+			if it.Quantity != 1 {
+				t.Errorf("parent qty = %d, want 1 (untouched)", it.Quantity)
+			}
+		default:
+			if it.Quantity != 2 {
+				t.Errorf("child qty = %d, want 2 (merged on same-PARENT re-add)", it.Quantity)
+			}
+		}
+	}
+}
+
+// Same ItemId+StoreId+both with ParentId set to *different* parent lines
+// must also stay distinct — children belong to their own parent.
+func TestAddItem_DoesNotMergeAcrossDifferentParentLines(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: "P1", Quantity: 1, Price: 500}); err != nil {
+		t.Fatalf("add parent A: %v", err)
+	}
+	parentA := g.Items[0].Id
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 200, Sku: "P2", Quantity: 1, Price: 500}); err != nil {
+		t.Fatalf("add parent B: %v", err)
+	}
+	parentB := g.Items[1].Id
+
+	// Two children of different parents sharing an ItemId — distinct lines.
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 999, Sku: "C", Quantity: 1, Price: 100, ParentId: &parentA}); err != nil {
+		t.Fatalf("add child of A: %v", err)
+	}
+	if _, err := reg.Apply(ctx, g, &cart_messages.AddItem{ItemId: 999, Sku: "C", Quantity: 1, Price: 100, ParentId: &parentB}); err != nil {
+		t.Fatalf("add child of B: %v", err)
+	}
+
+	// 2 parent lines + 2 child lines = 4 total; no merges across.
+	if len(g.Items) != 4 {
+		t.Errorf("items = %d, want 4 (parents A,B + their distinct children)", len(g.Items))
+	}
+	for _, it := range g.Items {
+		if it.Quantity != 1 {
+			t.Errorf("line %d qty = %d, want 1 (no accidental merge)", it.Id, it.Quantity)
+		}
+	}
+}