Bug: Flutter ObjectBox store cannot open when Kotlin model exists — UID mismatch

[AminSallah]

Description

When using ObjectBox in a Flutter app and also accessing the same database from Android (Kotlin) code (e.g., an AppWidget provider), the store fails to open due to an entity UID mismatch, even though both models share the same fields and types.

Both sides define the same entity Counter with id and value fields, but the generated UIDs differ between Dart’s objectbox.g.dart and Android’s MyObjectBox.java.

Dart error

ObjectBoxException: failed to create store:
Incoming entity ID 1:3702879537994877312 does not match existing UID 6372614592112598027

Even after removing the entire database folder and regenerating both models , the UIDs remain different.

---

Expected behavior

If both models have the same schema (id and value fields, same types, same entity name), the generated entity UIDs should match, allowing both the Dart and Android code to open the same BoxStore.

---

Actual behavior

UIDs differ between Dart and Kotlin generators, making it impossible to share the same database between Flutter and native Android code.

---

Dart model

import 'package:objectbox/objectbox.dart';
@Entity()

class Counter {

@Id()

int id = 0; // ObjectBox assigns id when 0

int value;
Counter({this.id = 0, this.value = 0});
@override

String toString() => 'Counter{id: $id, value: $value}';

}

Kotlin model

package com.example.object_box.model
import io.objectbox.annotation.Entity

import io.objectbox.annotation.Id
@Entity

data class Counter(

@Id var id: Long = 0,

var value: Int = 0

)

---

Generated UID comparison

Source	Entity UID	Property UIDs
Dart (objectbox.g.dart)	3702879537994877312	id=7081767577393141001, value=9081267074309558120
Kotlin (MyObjectBox.java)	6372614592112598027	id=3796890794073728955, value=6309849204992249866

---

Steps to reproduce

1. Create a Flutter app with objectbox and objectbox_flutter_libs.

2. Add a Kotlin model with the same fields and generate ObjectBox code.

3. Build both sides.

4. Use same database path and try to open the store in both Dart and Kotlin using the same directory.

5. UID mismatch and SchemaException found in flutter logs when first write .

---

Environment

Flutter:

Flutter 3.35.5
objectbox: ^5.0.0
objectbox_flutter_libs: ^5.0.0

Android:

objectbox-android: 5.0.0
objectbox-processor: 5.0.0

Reproducible: Always

---

Additional context

Even with identical schema and matching types (int ↔ Long), the entity UIDs are completely different. There appears to be no documented way to manually align UIDs or to instruct both generators to produce matching UIDs for the same model.

If Flutter and Android ObjectBox runtimes are not intended to share a store directly, please clarify this in the documentation — currently it’s not obvious that cross-language schema sharing is unsupported.

---

Is it possible for Flutter and Android (Java/Kotlin) to share the same ObjectBox database?

[greenrobot]

You would have to keep the JSON model files in sync and allow writes only from one app. Otherwise, this is not supported.

[AminSallah]

Even after I synced the UID in Dart's objectbox.g and the JSON model to match Java, I got this error:

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: ObjectBoxException: failed to create store: Existing Property value (2, Int) of entity Counter is not compatible with the new type Long

We need to read the database in native code to perform background tasks on the data, instead of initializing the Dart engine.

Java model

package com.example.object_box.model;

import io.objectbox.BoxStore;
import io.objectbox.BoxStoreBuilder;
import io.objectbox.ModelBuilder;
import io.objectbox.ModelBuilder.EntityBuilder;
import io.objectbox.model.PropertyFlags;
import io.objectbox.model.PropertyType;

// THIS CODE IS GENERATED BY ObjectBox, DO NOT EDIT.
/**
 * Starting point for working with your ObjectBox. All boxes are set up for your objects here.
 * <p>
 * First steps (Android): get a builder using {@link #builder()}, call {@link BoxStoreBuilder#androidContext(Object)},
 * and {@link BoxStoreBuilder#build()} to get a {@link BoxStore} to work with.
 */
public class MyObjectBox {

    public static BoxStoreBuilder builder() {
        BoxStoreBuilder builder = new BoxStoreBuilder(getModel());
        builder.entity(Counter_.__INSTANCE);
        return builder;
    }

    private static byte[] getModel() {
        ModelBuilder modelBuilder = new ModelBuilder();
        modelBuilder.lastEntityId(1, 6372614592112598027L);
        modelBuilder.lastIndexId(0, 0L);
        modelBuilder.lastRelationId(0, 0L);

        buildEntityCounter(modelBuilder);

        return modelBuilder.build();
    }

    private static void buildEntityCounter(ModelBuilder modelBuilder) {
        EntityBuilder entityBuilder = modelBuilder.entity("Counter");
        entityBuilder.id(1, 6372614592112598027L).lastPropertyId(2, 6309849204992249866L);

        entityBuilder.property("id", PropertyType.Long)
                .id(1, 3796890794073728955L)
                .flags(PropertyFlags.ID);
        entityBuilder.property("value", PropertyType.Int)
                .id(2, 6309849204992249866L);

        entityBuilder.entityDone();
    }


}

Dart model

// GENERATED CODE - DO NOT MODIFY BY HAND
// This code was generated by ObjectBox. To update it run the generator again
// with `dart run build_runner build`.
// See also https://docs.objectbox.io/getting-started#generate-objectbox-code

// ignore_for_file: camel_case_types, depend_on_referenced_packages
// coverage:ignore-file

import 'dart:typed_data';

import 'package:flat_buffers/flat_buffers.dart' as fb;
import 'package:objectbox/internal.dart'
    as obx_int; // generated code can access "internal" functionality
import 'package:objectbox/objectbox.dart' as obx;
import 'package:objectbox_flutter_libs/objectbox_flutter_libs.dart';

import 'model/counter.dart';

export 'package:objectbox/objectbox.dart'; // so that callers only have to import this file

final _entities = <obx_int.ModelEntity>[
  obx_int.ModelEntity(
    id: const obx_int.IdUid(1, 6372614592112598027),
    name: 'Counter',
    lastPropertyId: const obx_int.IdUid(2, 6309849204992249866),
    flags: 0,
    properties: <obx_int.ModelProperty>[
      obx_int.ModelProperty(
        id: const obx_int.IdUid(1, 3796890794073728955),
        name: 'id',
        type: 6,
        flags: 1,
      ),
      obx_int.ModelProperty(
        id: const obx_int.IdUid(2, 6309849204992249866),
        name: 'value',
        type: 6,
        flags: 0,
      ),
    ],
    relations: <obx_int.ModelRelation>[],
    backlinks: <obx_int.ModelBacklink>[],
  ),
];

/// Shortcut for [obx.Store.new] that passes [getObjectBoxModel] and for Flutter
/// apps by default a [directory] using `defaultStoreDirectory()` from the
/// ObjectBox Flutter library.
///
/// Note: for desktop apps it is recommended to specify a unique [directory].
///
/// See [obx.Store.new] for an explanation of all parameters.
///
/// For Flutter apps, also calls `loadObjectBoxLibraryAndroidCompat()` from
/// the ObjectBox Flutter library to fix loading the native ObjectBox library
/// on Android 6 and older.
Future<obx.Store> openStore({
  String? directory,
  int? maxDBSizeInKB,
  int? maxDataSizeInKB,
  int? fileMode,
  int? maxReaders,
  bool queriesCaseSensitiveDefault = true,
  String? macosApplicationGroup,
}) async {
  await loadObjectBoxLibraryAndroidCompat();
  return obx.Store(
    getObjectBoxModel(),
    directory: directory ?? (await defaultStoreDirectory()).path,
    maxDBSizeInKB: maxDBSizeInKB,
    maxDataSizeInKB: maxDataSizeInKB,
    fileMode: fileMode,
    maxReaders: maxReaders,
    queriesCaseSensitiveDefault: queriesCaseSensitiveDefault,
    macosApplicationGroup: macosApplicationGroup,
  );
}

/// Returns the ObjectBox model definition for this project for use with
/// [obx.Store.new].
obx_int.ModelDefinition getObjectBoxModel() {
  final model = obx_int.ModelInfo(
    entities: _entities,
    lastEntityId: const obx_int.IdUid(1, 6372614592112598027),
    lastIndexId: const obx_int.IdUid(0, 0),
    lastRelationId: const obx_int.IdUid(0, 0),
    lastSequenceId: const obx_int.IdUid(0, 0),
    retiredEntityUids: const [],
    retiredIndexUids: const [],
    retiredPropertyUids: const [],
    retiredRelationUids: const [],
    modelVersion: 5,
    modelVersionParserMinimum: 5,
    version: 1,
  );

  final bindings = <Type, obx_int.EntityDefinition>{
    Counter: obx_int.EntityDefinition<Counter>(
      model: _entities[0],
      toOneRelations: (Counter object) => [],
      toManyRelations: (Counter object) => {},
      getId: (Counter object) => object.id,
      setId: (Counter object, int id) {
        object.id = id;
      },
      objectToFB: (Counter object, fb.Builder fbb) {
        fbb.startTable(3);
        fbb.addInt64(0, object.id);
        fbb.addInt64(1, object.value);
        fbb.finish(fbb.endTable());
        return object.id;
      },
      objectFromFB: (obx.Store store, ByteData fbData) {
        final buffer = fb.BufferContext(fbData);
        final rootOffset = buffer.derefObject(0);
        final idParam = const fb.Int64Reader().vTableGet(
          buffer,
          rootOffset,
          4,
          0,
        );
        final valueParam = const fb.Int64Reader().vTableGet(
          buffer,
          rootOffset,
          6,
          0,
        );
        final object = Counter(id: idParam, value: valueParam);

        return object;
      },
    ),
  };

  return obx_int.ModelDefinition(model, bindings);
}

/// [Counter] entity fields to define ObjectBox queries.
class Counter_ {
  /// See [Counter.id].
  static final id = obx.QueryIntegerProperty<Counter>(
    _entities[0].properties[0],
  );

  /// See [Counter.value].
  static final value = obx.QueryIntegerProperty<Counter>(
    _entities[0].properties[1],
  );
}

[greenrobot]

The error is because the model is (or was) different. Please have a closer look at https://docs.objectbox.io/advanced/data-model-updates and https://docs.objectbox.io/advanced/meta-model-ids-and-uids to understand the context.

[AminSallah]

I've already read the documents; this is exactly what's happening:

1. I created the Counter model in Kotlin and built it.
2. Using the UIDs from the generated default.json, I created the Dart model with the exact same UIDs. Then I ran build_runner to generate the Dart JSON model. After the build, the generated objectbox-model.json matches the default.json produced by Java.

// Example Dart model mirroring the Java model
import 'package:objectbox/objectbox.dart';

@Entity(uid: 6559915653919869890) // obtained from default.json
class Counter {
  @Id()
  @Property(uid: 8786061352538829094)  // obtained from default.json
  int id = 0;

  @Property(uid: 6801141490862310639)  // obtained from default.json
  int value = 0;

  Counter({this.id = 0, this.value = 0});

  @override
  String toString() => 'Counter{id: $id, value: $value}';
}

3. The store is created using Android native code and I can perform operations on it, but when trying to open the store in Flutter I get this error:

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: ObjectBoxException: failed to create store: Existing Property value (2, Int) of entity Counter is not compatible with the new type Long

I think this is because the ID type in Java is long, while Dart uses int, even though the two models and the generated JSON files are identical.

[greenrobot]

I think this is because the ID type in Java is long, while Dart uses int

This should be fine, both are 64 bit.

Check the value property - is must also be long in Java otherwise you see the error because of the mismatch.

[AminSallah]

It’s Int in Java, and that’s what caused the error.
I’m not sure if this is a bug or intended behavior, but it worked like magic for me once I changed the property type from Int to long in the Java model (not the other way around). This fixed the issue, and now the same box can be accessed natively in Kotlin and Dart.

So, to make this work properly, any property of type Int in the Dart model must be mirrored as long in the Java model.

Thanks for your time! If this isn’t a bug, feel free to close the issue.