Скачать или смотреть Understanding ObjectBox ToMany: No Duplicates or Guaranteed Order in Relations

Learn why ObjectBox ToMany containers do not allow duplicates or maintain order, and discover practical approaches to model many-to-many relationships with ordering and duplicates.
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This video is based on the question https://stackoverflow.com/q/79456009/ asked by the user 'Eddie Jakobitz' ( https://stackoverflow.com/u/28664107/ ) and on the answer https://stackoverflow.com/a/79461064/ provided by the user 'Markus Junginger' ( https://stackoverflow.com/u/551269/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Challenge with ToMany in ObjectBox

When migrating from Realm to ObjectBox, developers often encounter surprising behavior using ToMany containers instead of Realm's RealmList. Specifically:

Duplicates are not allowed. After saving and restoring an object, duplicate entries in a ToMany relation are automatically removed.

Order is not maintained. The order of items added by the user is lost after database operations; restored lists are sorted by object ID rather than insertion order.

These limitations stem from how ObjectBox manages relations internally.

Why Does ToMany Behave This Way?

ObjectBox uses an index-based storage for relations rather than a list of IDs. This approach:

Ensures uniqueness of related objects—no duplicates.

Stores related objects in a highly optimized, unordered index for quick queries and updates.

While this maximizes performance and efficiency, it means ToMany does not fulfill the List contract fully with respect to duplicates and ordering.

How to Handle Duplicates and Ordering in ObjectBox

If your application needs many-to-many relationships that allow duplicates and require order preservation, consider these strategies:

Model the relationship as a separate entity with sequence data:

Create a new entity representing the link between two objects.

This entity holds two ToOne relations and an integer sequence field to record order.

Enables precise control over duplicates and ordering.

Request a new list-based relation feature:

Engage with ObjectBox developers or community to propose a native list relation that inherently supports duplicates and order.

Such a feature could handle ID mapping during synchronization automatically.

Maintain a separate list property for order and duplicates:

Store identifiers or secondary IDs in a separate property (e.g., a list of custom keys).

Be cautious with synchronization; ObjectBox IDs are device-specific and may not be consistent across devices.

Summary

While ToMany relations in ObjectBox are efficient and integral for most use cases, they do not support duplicates or maintain insertion order due to their underlying implementation.

For scenarios needing ordered collections and duplicates, modeling relationships explicitly as entities with sequence metadata is the most robust solution today.

This approach aligns your data model with ObjectBox’s strengths while meeting your application’s domain requirements.



Further Reading

ObjectBox Relations documentation

Designing many-to-many relationships with order