fixes

pkg/cart/cart_item_json.go

@@ -0,0 +1,77 @@
+package cart
+
+import (
+	"encoding/json"
+	"reflect"
+	"strings"
+)
+
+// cartItemAlias mirrors CartItem but has no custom (Un)MarshalJSON, so it is
+// used to (de)serialize the typed fields without recursing.
+type cartItemAlias CartItem
+
+// knownItemKeys is the set of top-level JSON object keys owned by typed
+// CartItem fields. Any other key in an item object is dynamic and kept in
+// CartItem.Extra.
+var knownItemKeys = buildKnownItemKeys()
+
+func buildKnownItemKeys() map[string]struct{} {
+	keys := make(map[string]struct{})
+	t := reflect.TypeOf(CartItem{})
+	for i := 0; i < t.NumField(); i++ {
+		name, _, _ := strings.Cut(t.Field(i).Tag.Get("json"), ",")
+		if name == "" {
+			name = t.Field(i).Name // untagged fields serialize under their Go name
+		}
+		if name == "-" {
+			continue // e.g. Extra itself
+		}
+		keys[name] = struct{}{}
+	}
+	return keys
+}
+
+// MarshalJSON emits the typed fields and flattens Extra onto the same object.
+// Typed fields take precedence on key collisions.
+func (c CartItem) MarshalJSON() ([]byte, error) {
+	core, err := json.Marshal(cartItemAlias(c))
+	if err != nil || len(c.Extra) == 0 {
+		return core, err
+	}
+	merged := make(map[string]json.RawMessage, len(c.Extra)+8)
+	if err := json.Unmarshal(core, &merged); err != nil {
+		return nil, err
+	}
+	for k, v := range c.Extra {
+		if _, taken := merged[k]; taken {
+			continue
+		}
+		merged[k] = v
+	}
+	return json.Marshal(merged)
+}
+
+// UnmarshalJSON reads the typed fields and collects every other key into Extra.
+func (c *CartItem) UnmarshalJSON(data []byte) error {
+	var alias cartItemAlias
+	if err := json.Unmarshal(data, &alias); err != nil {
+		return err
+	}
+	*c = CartItem(alias)
+
+	all := make(map[string]json.RawMessage)
+	if err := json.Unmarshal(data, &all); err != nil {
+		return err
+	}
+	for k := range all {
+		if _, known := knownItemKeys[k]; known {
+			delete(all, k)
+		}
+	}
+	if len(all) > 0 {
+		c.Extra = all
+	} else {
+		c.Extra = nil
+	}
+	return nil
+}