ObjectBox manages its data model (schema) mostly automatically. The data model is defined by the Entities you define in FlatBuffer schema files. When you add or remove entities or properties of your entities, ObjectBox takes care of those changes when you launch objectbox-generator on the changed .fbs file without any further action needed from you.

For other changes like renaming or changing the type, ObjectBox needs extra information to make things unambiguous. This works using unique identifiers (UIDs) specified by the uid annotation, as we will see below.

Renaming Entities and Properties

So why do we need that UID annotation? If you simply rename an entity, 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 applies when renaming properties. We'll show an example for both an entity and a property rename.

Step 1: Add an empty objectbox:uid annotation to the entity/property you want to rename:

schema.fbs
/// objectbox:uid
table OldEntityName {
    id: ulong;
}
​
table Task {
    id: ulong;
    /// objectbox:uid
    old_property_name: string;
}

Step 2: Re-generate ObjectBox code for the file using objectbox-generator [-c|-cpp] schema.fbs. The generation will fail with an error message that gives you the current UID of the entity/property:

output for empty "uid" annotation on an entity
can't merge model information: entity OldEntityName: uid annotation value must 
not be empty (model entity UID = 8427767268318374108) on entity OldEntityName

output for empty "uid" annotation on a property
can't merge model information: merging entity Task: property old_property_name: 
uid annotation value must not be empty:
    [rename] apply the current UID 7671517997700696229
    [change/reset] apply a new UID 2346823240790844685

Step 3: Apply the UID printed in the error message to your entity/property:

/// objectbox: uid=8427767268318374108
table OldEntityName {
    id: ulong;
}
​
table Task {
    id: ulong;
    /// objectbox:uid=7671517997700696229
    old_property_name: string;
}

Step 4: The last thing to do is the actual rename on the language level:

/// objectbox: uid=8427767268318374108
table RenamedEntity {
    id: ulong;
}
​
table Task {
    id: ulong;
    /// objectbox:uid=7671517997700696229
    renamed_property: string;
}

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

Note: Instead of the above you can also find the UID of the entity/property in the objectbox-model.json file yourself and add it as a value of the uid annotation when renaming your entity/property to achieve the same effect.

Changing Property Types

New property, different name

It's just adding an additional property - this solution useful if you need to do the data migration manually or just want to keep the old data around.

table Task {
    id: ulong;
    old_property: string;
}
​
// becomes
​
table Task {
    id: ulong;
    old_property: string;
    new_property: int;
}
​
// Note, if the property already had an UID annotation, 
//  don't add the same UID to the new property - skip the annotation instead.

Reset data - new UID on a property

This solution useful if you don't care about the original data at all = it will be lost.

Step 1: Add an empty `objectbox:"uid"` annotation to the property you want to reset:

table Task {
    id: ulong;
    /// objectbox:uid
    text: string;
}

Step 2: Re-generate ObjectBox code for the project using go generate ./... in your project directory. The generation will fail with an error message that gives you the current UID of the property:

can't merge model information: merging entity Task: property text: 
uid annotation value must not be empty:
    [rename] apply the current UID 7671517997700696229
    [change/reset] apply a new UID 2346823240790844685

Step 3: Apply the UID printed in the error message to your property (and change its type):

table Task {
    id: ulong;
    /// objectbox: uid=2346823240790844685
    text: string;
}

You can now use the property in your entity as if it was a new one (no data stored)