Entity Inheritance
How to inherit properties from entity super classes.

ObjectBox - Entity Super Classes

Only available for Java/Kotlin at the moment
ObjectBox allows entity inheritance to share persisted properties in super classes. The base class can be an entity or non-entity class. For this purpose the @Entity annotation is complemented by the @BaseEntity annotation. There are three types of super classes, which are defined via annotations:
  • No annotation: The base class and its properties are not considered for persistence.
  • @BaseEntity: Properties are considered for persistence in sub classes, but the base class itself cannot be persisted.
  • @Entity: Properties are considered for persistence in sub classes, and the base class itself is a normally persisted entity.
For example:
Java
Kotlin
// Superclass:
@BaseEntity
public abstract class Base {
@Id long id;
String baseString;
public Base() {
}
public Base(long id, String baseString) {
this.id = id;
this.baseString = baseString;
}
}
// Subclass:
@Entity
public class Sub extends Base {
String subString;
public Sub() {
}
public Sub(long id, String baseString, String subString) {
super(id, baseString);
this.subString = subString;
}
}
// Note: Kotlin data classes do not support inheritance,
// so this example uses regular Kotlin classes.
// Superclass:
@BaseEntity
abstract class Base {
@Id
var id: Long = 0
var baseString: String? = null
constructor()
constructor(id: Long, baseString: String?) {
this.id = id
this.baseString = baseString
}
}
// Subclass:
@Entity
class Sub : Base {
var subString: String? = null
constructor()
constructor(id: Long,
baseString: String?,
subString: String?) : super(id, baseString) {
this.subString = subString
}
}
The model for Sub, Sub_, will now include all properties: id , baseString and subString .
It is also possible to inherit properties from another entity:
Java
Kotlin
// Entities inherit properties from super entities.
@Entity
public class SubSub extends Sub {
String subSubString;
public SubSub() {
}
public SubSub(long id, String baseString,
String subString, String subSubString) {
super(id, baseString, subString);
this.subSubString = subSubString;
}
}
// Entities inherit properties from super entities.
@Entity
class SubSub : Sub {
var subSubString: String? = null
constructor()
constructor(id: Long,
baseString: String?,
subString: String?,
subSubString: String?) : super(id, baseString, subString) {
this.subSubString= subSubString
}
}

Notes on usage

  • It is possible to have classes in the inheritance chain that are not annotated with @BaseEntity. Their properties will be ignored and will not become part of the entity model.
  • It is not generally recommend to have a base entity class consisting of an ID property only. E.g. Java imposes an additional overhead to construct objects with a sub class.
  • Depending on your use case using interfaces may be more straightforward.

Restrictions

  • Superclasses annotated with @BaseEntity can not be part of a library.
  • There are no polymorphic queries (e.g. you cannot query for a base class and expect results from sub classes).
  • Currently any superclass, whether it is an @Entity or @BaseEntity, can not have any relations (like a ToOne or ToMany property).
Java
Kotlin
// THIS DOES NOT WORK
@BaseEntity
public abstract class Base {
@Id long id;
ToOne<OtherEntity> other;
ToMany<OtherEntity> others;
}
// THIS DOES NOT WORK
@BaseEntity
abstract class Base {
@Id
var id: Long = 0
lateinit var other: ToOne<OtherEntity>
lateinit var others: ToMany<OtherEntity>
}