Make Relationship generic

Author: f3ath

lib/src/document/many.dart

@@ -1,7 +1,7 @@
 import 'package:json_api/src/document/identifier.dart';
 import 'package:json_api/src/document/relationship.dart';
 
-class ToMany extends Relationship {
+class ToMany extends Relationship<Identifier> {
   ToMany(Iterable<Identifier> identifiers) {
     _ids.addAll(identifiers);
   }

lib/src/document/one.dart

@@ -1,7 +1,7 @@
 import 'package:json_api/src/document/identifier.dart';
 import 'package:json_api/src/document/relationship.dart';
 
-class ToOne extends Relationship {
+class ToOne extends Relationship<Identifier> {
   ToOne(this.identifier);
 
   ToOne.empty() : this(null);
@@ -13,5 +13,5 @@ class ToOne extends Relationship {
 
   @override
   Iterator<Identifier> get iterator =>
-      identifier == null ? <Identifier>[].iterator : [identifier!].iterator;
+      identifier == null ? super.iterator : [identifier!].iterator;
 }

lib/src/document/relationship.dart

@@ -1,9 +1,8 @@
 import 'dart:collection';
 
-import 'package:json_api/src/document/identifier.dart';
 import 'package:json_api/src/document/link.dart';
 
-class Relationship with IterableMixin<Identifier> {
+class Relationship<T> with IterableMixin<T> {
   final links = <String, Link>{};
   final meta = <String, Object?>{};
 
@@ -13,5 +12,5 @@ class Relationship with IterableMixin<Identifier> {
       };
 
   @override
-  Iterator<Identifier> get iterator => const <Identifier>[].iterator;
+  Iterator<T> get iterator => <T>[].iterator;
 }