Data Model Updates

ObjectBox - Data Model Updates

ObjectBox manages its data model (schema) mostly automatically. The data model is defined by the entity classes you define. When you add or remove entities or properties of your entities, ObjectBox takes care of those changes without any further action from you.

For other changes like renaming or changing the type, ObjectBox needs extra information to make things unambiguous. This works using unique IDs (UIDs) and an @Uid annotation, as we will see below.


ObjectBox keeps track of entities and properties by assigning them unique IDs (UIDs). All those UIDs are stored in a file “objectbox-models/default.json”, which you should add to your version control system (e.g. git). If you are interested, we have in-depth documentation on UIDs and concepts. But let’s continue with how to rename entities or properties.

In short: To make UID-related changes, put an @Uid annotation on the entity or property and build the project to get further instructions. Repeat for each entity or property to change.

Renaming Entities and Properties

So why do we need that UID annotation? If you simply rename an entity class, ObjectBox only sees that the old entity is gone and a new entity is available. This can be interpreted in two ways:

  • The old entity is removed and a new entity should be added, the old data is discarded. This is the default behavior of ObjectBox.

  • The entity was renamed, the old data should be re-used.

So to tell ObjectBox to do a rename instead of discarding your old entity and data, you need to make sure it knows that this is the same entity and not a new one. You do that by attaching the internal UID to the entity.

The same is true for properties.

Now let’s walk through how to do that. The process works the same if you want to rename a property:

How-to and Example

Step 1: Add an empty @Uid annotation to the entity/property you want to rename:

public class MyName { ... }

Step 2: Build the project. The build will fail with an error message that gives you the current UID of the entity/property:

error: [ObjectBox] UID operations for entity "MyName":
[Rename] apply the current UID using @Uid(6645479796472661392L) -
[Change/reset] apply a new UID using @Uid(4385203238808477712L)

Step 3: Apply the UID from the [Rename] section of the error message to your entity/property:

public class MyName { ... }

Step 4: The last thing to do is the actual rename on the language level (Java, Kotlin, etc.):

public class MyNewName { ... }

You can now use your renamed entity/property as expected and all existing data will still be there.

Repeat the steps above to rename another entity or property.

Note: Instead of the above you can also find the UID of the entity/property in the ObjectBox default.json file yourself and add it together with the @Uid annotation before renaming your entity/property. This can be faster when renaming multiple properties.

Changing Property Types

ObjectBox does not support migrating existing property data to a new type. You will have to take care of this yourself, e.g. by keeping the old property and adding some migration logic.

There are two solutions to changing the type of a property:

  • Add a new property with a different name (this only works if the property has no @Uid annotation already):

// old:
String year;
// new:
int yearInt;
  • Set a new UID for the property so ObjectBox treats it as a new property. Let’s walk through how to do that:

How-to and Example

Step 1: Add the @Uid annotation to the property where you want to change the type:

String year;

Step 2: Build the project. The build will fail with an error message that gives you a newly created UID value:

error: [ObjectBox] UID operations for property "MyEntity.year":
[Rename] apply the current UID using @Uid(6707341922395832766L) -
[Change/reset] apply a new UID using @Uid(9204131405652381067L)

Step 3: Apply the UID from the [Change/reset] section to your property:

int year;

You can now use the property in your entity as if it were a new one.

Repeat the steps above to change the type of another property.