fix: TypeError on addRelation function (#1098)

Author: Copilot

packages/dart/lib/src/objects/parse_relation.dart

@@ -61,7 +61,7 @@ class _ParseRelation<T extends ParseObject>
 
   // For offline caching, we keep track of every object
   // we've known to be in the relation.
-  Set<T> knownObjects = <T>{};
+  Set<ParseObject> knownObjects = <ParseObject>{};
 
   _ParseRelationOperation? lastPreformedOperation;
 
@@ -90,7 +90,7 @@ class _ParseRelation<T extends ParseObject>
 
     lastPreformedOperation = relationOperation;
 
-    knownObjects = lastPreformedOperation!.value.toSet() as Set<T>;
+    knownObjects = lastPreformedOperation!.value.toSet();
 
     return this;
   }

packages/dart/test/src/objects/parse_object/parse_object_relation_test.dart

@@ -624,5 +624,137 @@ void main() {
       // assert
       expect(() => getRelation(), throwsA(isA<ParseRelationException>()));
     });
+
+    test('addRelation() should work with custom ParseObject subclasses', () {
+      // arrange
+      // Create custom ParseObject subclasses similar to the issue report
+      final contact1 = Contact()..objectId = 'contact1';
+      final contact2 = Contact()..objectId = 'contact2';
+
+      final order = Order();
+
+      // act & assert
+      // This should not throw a TypeError
+      expect(
+        () => order.addRelation('receivers', [contact1, contact2]),
+        returnsNormally,
+      );
+
+      final toJsonAfterAddRelation = order.toJson(forApiRQ: true);
+
+      const expectedToJson = {
+        "receivers": {
+          "__op": "AddRelation",
+          "objects": [
+            {
+              "__type": "Pointer",
+              "className": "Contact",
+              "objectId": "contact1",
+            },
+            {
+              "__type": "Pointer",
+              "className": "Contact",
+              "objectId": "contact2",
+            },
+          ],
+        },
+      };
+
+      expect(
+        DeepCollectionEquality().equals(expectedToJson, toJsonAfterAddRelation),
+        isTrue,
+      );
+    });
+
+    test(
+      'addRelation() should work when getRelation<T>() was called first with typed generic',
+      () {
+        // This test reproduces issue #999
+        // The issue occurs when:
+        // 1. getRelation<Contact>() is called first (creating _ParseRelation<Contact>)
+        // 2. Then addRelation() is called with Contact objects
+        // 3. The merge operation creates a Set<ParseObject>
+        // 4. Trying to cast Set<ParseObject> to Set<Contact> throws TypeError
+
+        // arrange
+        final contact1 = Contact()..objectId = 'contact1';
+        final contact2 = Contact()..objectId = 'contact2';
+
+        final order = Order();
+
+        // First, get the relation with typed generic (this creates _ParseRelation<Contact>)
+        order.getRelation<Contact>('receivers');
+
+        // act & assert
+        // This should NOT throw: _TypeError (type '_Set<ParseObject>' is not a subtype of type 'Set<Contact>' in type cast)
+        expect(
+          () => order.addRelation('receivers', [contact1, contact2]),
+          returnsNormally,
+        );
+      },
+    );
+
+    test(
+      'calling addRelation() multiple times with custom subclasses should work',
+      () {
+        // arrange
+        final contact1 = Contact()..objectId = 'contact1';
+        final contact2 = Contact()..objectId = 'contact2';
+        final contact3 = Contact()..objectId = 'contact3';
+
+        final order = Order();
+
+        // act & assert
+        // First addRelation call
+        expect(
+          () => order.addRelation('receivers', [contact1]),
+          returnsNormally,
+        );
+
+        // Second addRelation call - this should also not throw
+        expect(
+          () => order.addRelation('receivers', [contact2, contact3]),
+          returnsNormally,
+        );
+      },
+    );
+
+    test('removeRelation() should work with custom ParseObject subclasses', () {
+      // arrange
+      final contact1 = Contact()..objectId = 'contact1';
+      final contact2 = Contact()..objectId = 'contact2';
+
+      final order = Order();
+
+      // act & assert
+      expect(
+        () => order.removeRelation('receivers', [contact1, contact2]),
+        returnsNormally,
+      );
+    });
   });
 }
+
+/// Custom ParseObject subclass for testing (similar to the issue report)
+class Contact extends ParseObject implements ParseCloneable {
+  Contact() : super(_keyTableName);
+  Contact.clone() : this();
+
+  @override
+  clone(Map<String, dynamic> map) =>
+      Contact.clone()..fromJson(Map<String, dynamic>.from(map));
+
+  static const String _keyTableName = 'Contact';
+}
+
+/// Custom ParseObject subclass for testing (similar to the issue report)
+class Order extends ParseObject implements ParseCloneable {
+  Order() : super(_keyTableName);
+  Order.clone() : this();
+
+  @override
+  clone(Map<String, dynamic> map) =>
+      Order.clone()..fromJson(Map<String, dynamic>.from(map));
+
+  static const String _keyTableName = 'Order';
+}