fixes

pkg/cart/mutation_set_custom_fields.go

@@ -0,0 +1,31 @@
+package cart
+
+import (
+	"fmt"
+
+	messages "git.k6n.net/go-cart-actor/proto/cart"
+)
+
+// SetLineItemCustomFields sets/merges user-supplied custom input fields on an
+// existing cart line. It is the dict equivalent of LineItemMarking: keys in the
+// request are upserted; existing keys not present in the request are left
+// untouched. (Send an empty value to clear a single field at the API layer if
+// desired.)
+func SetLineItemCustomFields(grain *CartGrain, req *messages.SetLineItemCustomFields) error {
+	for _, item := range grain.Items {
+		if item.Id != req.Id {
+			continue
+		}
+		if len(req.CustomFields) == 0 {
+			return nil
+		}
+		if item.CustomFields == nil {
+			item.CustomFields = make(map[string]string, len(req.CustomFields))
+		}
+		for k, v := range req.CustomFields {
+			item.CustomFields[k] = v
+		}
+		return nil
+	}
+	return fmt.Errorf("item with ID %d not found", req.Id)
+}