# 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 `// objectbox: uid` annotation, as we will see below.

## UIDs

ObjectBox keeps track of entities and properties by assigning them unique IDs (UIDs). All those UIDs are stored in a file “model.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](/advanced/meta-model-ids-and-uids.md). But let’s continue with how to rename entities or properties.

**In short:** To make UID-related changes, put an  `// objectbox: uid` annotation on the entity or property and build the project to get further instructions.

## 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 `// objectbox: uid`  annotation to the entity/property you want to rename:

```swift
// objectbox: entity
// objectbox: uid
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: No UID given for entity MyName. You can do the following:
	[Rename] Apply the current UID using // objectbox: uid = 17664
	[Change/Reset] Apply a new UID using // objectbox: uid = 18688

```

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

```swift
// objectbox: entity
// objectbox: uid = 17664
class MyName { ... }
```

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

```java
// objectbox: entity
// objectbox: uid = 17664
class MyNewName { ... }
```

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

{% hint style="info" %}
 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 together with the `// objectbox: uid` annotation before renaming your entity/property.
{% endhint %}

## Changing Property Types

When you want to change the type of a property, you must tell ObjectBox to create a new property internally. This is because ObjectBox cannot migrate your data, so simply changing the type may lead to data loss. You can do this in two ways:

* Just rename the property, so it is treated as a new property (this only works if the property has no `// objectbox: uid` annotation already):

```swift
// old:
var year: String
// new:
var yearInt: Int
```

* Tell ObjectBox to use a new UID for the property. Let’s walk through how to do that:

#### How-to and Example

**Step 1:** Add the  `// objectbox: uid` annotation to the property where you want to change the type:

```swift
// objectbox: uid
var year: String
```

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

```
error: No UID given for property year of entity MyName. You can do the following:
	[Rename] Apply the current UID using // objectbox: uid = 15616
	[Change/Reset] Apply a new UID using // objectbox: uid = 18688
```

 **Step 3:** Apply the UID from the \[Change/Reset] section to your property:

```swift
// objectbox: uid = 18688
var year: Int
```

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


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