[BUG] `isar_generator` Crashes with "Null check operator used on a null value" for Composite Index

[tomkingroy]

[BUG] isar_generator Crashes with "Null check operator used on a null value" for Composite Index

Description

When using a composite index in an Isar collection with fields like int and DateTime, the isar_generator fails during code generation with the error:

[SEVERE] isar:isar_generator on lib/db/models/chat_message/chat_message.dart (cached):
Null check operator used on a null value

This bug prevents the generation of the *.g.dart file, blocking the use of the collection. The issue appears related to the composite index definition, as removing the composite index allows the generator to succeed. This behavior suggests a parsing bug in isar_generator when handling composite indexes, possibly involving late fields or specific field types.

Steps to Reproduce

1. Create a Flutter project with Isar dependencies (isar: ^4.0.3, isar_flutter_libs: ^4.0.3).
2. Define a collection with a composite index on an int field referencing a DateTime field.
3. Run dart run build_runner build to generate the *.g.dart file.
4. Observe the build failure with the error: Null check operator used on a null value.

Code Sample

import 'package:isar/isar.dart';

part 'chat_message.g.dart';

@collection
class ChatMessage {
  @Id()
  late int id;
  late String content;
  @Index(composite: [CompositeIndex('createdAt')])
  late int sessionId;
  late DateTime createdAt;
  late DateTime updatedAt;
}

pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  isar:
    hosted: https://pub.isar-community.dev
    version: ^4.0.3
  isar_flutter_libs:
    hosted: https://pub.isar-community.dev
    version: ^4.0.3
  path_provider: ^2.1.5

dev_dependencies:
  flutter_test:
    sdk: flutter
  build_runner: ^2.4.15

Run

flutter clean
flutter pub get
dart run build_runner build

Expected Behavior

• The isar_generator should successfully generate chat_message.g.dart without errors.
• The composite index (sessionId, createdAt) should be created, enabling efficient queries like:

isar.chatMessages
    .filter()
    .sessionIdEqualTo(123)
    .sortByCreatedAtDesc()
    .findAll();

Actual Behavior

Build fails with:

[SEVERE] isar:isar_generator on lib/db/models/chat_message/chat_message.dart (cached):
Null check operator used on a null value
[SEVERE] Failed after 99ms

No chat_message.g.dart is generated, preventing database usage.

Workarounds

• Remove composite index:

  @Index()
  late int sessionId;

  This allows generation but loses composite index benefits.

• Use single indexes:

  @Index()
  late int sessionId;
  @Index()
  late DateTime createdAt;

  This works but is less efficient for sorting queries.

• Remove late modifiers (not viable due to null safety errors without initialization)

Environment

• Isar Version: 4.0.3 (https://pub.isar-community.dev)
• Dart SDK: 3.7.2
• Flutter Version: 3.29.3
• Operating System: macOS 15.5 (24F74)
• Build Runner: 2.4.15

[tomkingroy]

I checked the type definitions for composite in version 4, and they are not like the ones shown in the official documentation’s demo. 😢

Writing it like this allows the build to succeed:

@Index(composite: ['createdAt'])