ObjectBox - Relations

Prefer to dive right into code? Check the relation project in the example repo.

Objects may reference other objects, for example using a simple reference or a list of objects. In database terms, we call those references relations. The object defining a relation we call the source object, the referenced object we call the target object. So a relation has a direction.

If there is one target object, we call the relation to-one. And if there can be multiple target objects, we call it to-many. Relations are lazily initialized: the actual target objects are fetched from the database when they are first accessed. Once the target objects are fetched, they are cached for further accesses.

To-One Relations

You define a to-one relation using the ToOne class, a smart proxy to the target object. It gets and caches the target object transparently. For example, an order is typically made by one customer. Thus, we could model the Order class to have a to-one relation to the Customer like this:

// Customer.java
@Entity
public class Customer {
    
    @Id public long id;
    
}
​
// Order.java
@Entity
public class Order {
    
    @Id public long id;
    
    public ToOne<Customer> customer;
    
}

@Entity
data class Customer(
        @Id var id: Long = 0
)
​
@Entity
data class Order(
        @Id var id: Long = 0
) {
    lateinit var customer: ToOne<Customer>
}

@Entity()
class Customer {
  int id;
}
​
@Entity()
class Order {
  int id;
  
  final customer = ToOne<Customer>();
}

Now let’s add a customer and some orders. To set the related customer object, call setTarget() (or assign target in Kotlin) on the ToOne instance and put the order object:

Customer customer = new Customer();
Order order = new Order();
order.customer.setTarget(customer);
// Puts order and customer:
long orderId = boxStore.boxFor(Order.class).put(order);

val customer = Customer()
val order = Order()
order.customer.target = customer
// Puts order and customer:
val orderId = boxStore.boxFor(Order::class.java).put(order)

final customer = Customer();
final order = Order();
​
// set the relation
order.customer.target = customer;
// Or you could create the target object in place:
// order.customer.target = Customer();
​
// Save the order and customer to the database
int orderId = store.box<Order>().put(order);

If the customer object does not yet exist in the database (i.e. its ID is zero), ToOne will put it (so there will be two puts, one for Order, one for Customer). If it already exists, ToOne will only create the relation (so there's only one put for Order, as explicitly written in the code). See further below for details about updating relations.

Have a look at the following code how you can get (read) the customer of an order:

Order order = boxStore.boxFor(Order.class).get(orderId);
Customer customer = order.customer.getTarget();

val order = boxStore.boxFor(Order::class.java)[orderId]
val customer = order.customer.target

final order = store.box<Order>().get(orderId);
final customer = order.customer.target;

This will do a database call on the first access (lazy loading). It uses lookup by ID, which is very fast in ObjectBox. If you only need the ID instead of the whole target object, you can completely avoid this database operation because it's already loaded: use order.customer.targetId/getTargetId().

We can also remove the relationship to a customer:

order.customer.setTarget(null);
boxStore.boxFor(Order.class).put(order);

order.customer.target = null
boxStore.boxFor(Order::class.java).put(order)

order.customer.target = null; // same as  .targetId = 0;
store.box<Order>().put(order);

Note that this does not remove the customer from the database, it just dissolves the relationship.

How ToOne works behind the scenes

If you look at your model in objectbox-models/default.json (or lib/bjectbox-model.json in Dart) you can see, a ToOne property is not actually stored. Instead, the ID of the target object is saved in a virtual property named like the ToOne property appended with Id.

Expose the ToOne target ID property

You can directly access the target ID property by defining a long (or Long in Kotlin) property in your entity class with the expected name:

@Entity
public class Order {
    @Id public long id;
    
    public long customerId; // ToOne target ID property
    public ToOne<Customer> customer;
}

@Entity
data class Order(
        @Id var id: Long = 0,
        var customerId: Long = 0
) {
    lateinit var customer: ToOne<Customer>
}

You can change the name of the expected target ID property by adding the @TargetIdProperty(String) annotation to a ToOne.

Initialization Magic

Did you notice that the ToOne field customer was never initialized in the code example above? Why can the code still use customer without any NullPointerException? Because the field actually is initialized – the initialization code just is not visible in your sources.

The ObjectBox Gradle plugin will transform your entity class (only supported for plain Java and Android projects) to do the proper initialization in constructors before your code is executed. Thus, even in your constructor code, you can just assume ToOne and ToMany/ List properties have been initialized and are ready for you to use.

@Entity
public class Example {
​    
    ToOne<Order> order;   
    ToMany<Order> orders;
 
    // Add a BoxStore field.
    transient BoxStore __boxStore;
    
    public Example() {
        // Initialize ToOne and ToMany
        this.order = new ToOne<>(this, Example_.order);
        this.orders = new ToMany<>(this, Example_.orders);
    }
    
    public Example(String value) {
        this();
        // Calls above constructor, so no need to initialize again.
    }
    
}

@Entity
class Example() {
​
    // Initialize ToOne and ToMany
    var order = ToOne<Order>(this, Example_.order)   
    var orders = ToMany<Order>(this, Example_.orders)
​
    // Add BoxStore field
    @JvmField
    @Transient
    @Suppress("PropertyName")
    var __boxStore: BoxStore? = null
​
}

Improve Performance

To improve performance when ObjectBox constructs your entities, you should provide an all-properties constructor.

For a ToOne you have to add an id parameter, typically named like the ToOne field appended with Id . Check your objectbox-models/default.json file to find the correct name.

An example:

@Entity
public class Order {
    
    @Id public long id;
    
    public ToOne<Customer> customer;
    
    public Order() { /* default constructor */ }
    
    public Order(long id, long customerId /* virtual ToOne id property */) {
        this.id = id;
        this.customer.setTargetId(customerId);
    }
    
}

To-Many Relations

To define a to-many relation, you can use a property of type ToMany. As the ToOne class, the ToMany class helps you to keep track of changes and to apply them to the database.

Note that to-many relations are resolved lazily on first access, and then cached in the source entity inside the ToMany object. So subsequent calls to any method, like size() of the ToMany, do not query the database, even if the relation was changed elsewhere. To get the latest data fetch the source entity again or call reset() on the ToMany.

There is a slight difference if you require a one-to-many (1:N) or many-to-many (N:M) relation. A 1:N relation is like the example above where a customer can have multiple orders, but an order is only associated with a single customer. An example for an N:M relation is students and teachers: students can have classes by several teachers but a teacher can also instruct several students.

One-to-Many (1:N)

To define a one-to-many relation, you need to annotate your relation property with @Backlink. It links back to a to-one relation in the target object. Using the customer and orders example, we can modify the customer class to have a to-many relation to the customer's orders:

// Customer.java
@Entity
public class Customer {
    
    @Id public long id;
    
    // 'to' is optional if only one relation matches.
    @Backlink(to = "customer")
    public ToMany<Order> orders;
    
}
​
// Order.java
@Entity
public class Order {
    
    @Id public long id;
    
    public ToOne<Customer> customer;
    
}

@Entity
data class Customer(
        @Id var id: Long = 0
) {    
    // 'to' is optional if only one relation matches.
    @Backlink(to = "customer")
    lateinit var orders: ToMany<Order>
}
​
@Entity
data class Order(
        @Id var id: Long = 0
) {
    lateinit var customer: ToOne<Customer>
}

@Entity()
class Customer {
  int id;
​
  // target-field argument is optional if only one relation matches.
  @Backlink('customer')
  final orders = ToMany<Order>();
}
​
@Entity()
class Order {
  int id;
​
  final customer = ToOne<Customer>();
}

Let’s add some orders together with a new customer. ToMany implements the Java List interface, so we can simply add orders to it:

Customer customer = new Customer();
customer.orders.add(new Order("Order 1"));
customer.orders.add(new Order("Order 2"));
// Puts customer and orders:
long customerId = boxStore.boxFor(Customer.class).put(customer);

val customer = Customer()
customer.orders.add(Order(text = "Order 1"))
customer.orders.add(Order(text = "Order 2"))
// Puts customer and orders:
val customerId = boxStore.boxFor(Customer::class.java).put(customer)

Customer customer = Customer();
customer.orders.add(Order('Order 1'));
customer.orders.add(Order('Order 2'));
// Puts customer and orders:
final customerId = store.box<Customer>().put(customer);

If the order entities do not yet exist in the database, ToMany will put them. If they already exist, it will only create the relation (but not put them). See further below for details about updating relations.

We can easily get the orders of a customer back by accessing the list of orders:

Customer customer = boxStore.boxFor(Customer.class).get(customerId);
for (Order order : customer.orders) {
    // Do something with each order.
}

val customer = boxStore.boxFor(Customer::class.java).get(customerId)
for (order in customer.orders) {
    // Do something with each order.
}

Customer customer = store.box<Customer>().get(customerId);
// you can use any List<> functions, for example:
final valueSum = customer.orders.fold(0, (sum, order) => sum + order.value);
// though you could use property queries and their .sum() function for that

Removing orders from the relation works as expected:

// Remove the relation to the first order in the list
Order order = customer.orders.remove(0);
boxStore.boxFor(Customer.class).put(customer);
// Optional: also remove the order entity from its box:
// boxStore.boxFor(Order.class).remove(order);

// Remove the relation to the first order in the list
val order = customer.orders.removeAt(0)
boxStore.boxFor(Customer::class.java).put(customer)
// Optional: also remove the order entity from its box:
// boxStore.boxFor(Order::class.java).remove(order)

// Remove the relation to the first order in the list
Order order = customer.orders.removeAt(0);
store.box<Customer>().put(customer);
// Optional: also remove the order entity from its box:
// store.box<Order>().remove(order);

Many-to-Many (N:M)

To define a many-to-many relation you simply add a property using the ToMany class. Assuming a students and teachers example, this is how a simple student class that has a to-many relation to teachers can look like:

// Teacher.java
@Entity
public class Teacher{
    
    @Id public long id;
    
}
​
// Student.java
@Entity
public class Student{
    
    @Id public long id;
    
    public ToMany<Teacher> teachers;
    
}

@Entity
data class Teacher(
        @Id var id: Long = 0
)
​
@Entity
data class Student(
        @Id var id: Long = 0
) {
    lateinit var teachers: ToMany<Teacher>
}

@Entity()
class Teacher{
  int id;
}
​
@Entity()
class Student{
  int id;
  
  final teachers = ToMany<Teacher>();
}

Adding the teachers of a student works exactly like with a list:

Teacher teacher1 = new Teacher();
Teacher teacher2 = new Teacher();
​
Student student1 = new Student();
student1.teachers.add(teacher1);
student1.teachers.add(teacher2);
​
Student student2 = new Student();
student2.teachers.add(teacher2);
​
// Puts students and teachers:
boxStore.boxFor(Student.class).put(student1, student2);

val teacher1 = Teacher()
val teacher2 = Teacher()
​
val student1 = Student()
student1.teachers.add(teacher1)
student1.teachers.add(teacher2)
​
val student2 = Student()
student2.teachers.add(teacher2)
​
// Puts students and teachers:
boxStore.boxFor(Student::class.java).put(student1, student2)

Teacher teacher1 = Teacher();
Teacher teacher2 = Teacher();
​
Student student1 = Student();
student1.teachers.add(teacher1);
student1.teachers.add(teacher2);
​
Student student2 = Student();
student2.teachers.add(teacher2);
​
// saves students as well as teachers in the database
store.box<Student>().putMany([student1, student2]);

If the teacher entities do not yet exist in the database, ToMany will also put them. If they already exist, ToMany will only create the relation (but not put them). See further below for details about updating relations.

To get the teachers of a student we just access the list:

Student student = boxStore.boxFor(Student.class).get(studentId);
for (Teacher teacher : student.teachers) {
    // Do something with each teacher.
}

val student = boxStore.boxFor(Student::class.java).get(studentId)
for (teacher in student.teachers) {
    // Do something with each teacher.
}

Student student = store.box<Student>().get(studentId);
// you can use any List<> functions, for example:
student.teachers.forEach((Teacher teacher) => ...);

And if a student drops out of a class, we can remove a teacher from the relation:

student.teachers.remove(0);
// Simply put the student again:
// boxStore.boxFor(Student.class).put(student);
// Or more efficient (than writing the thole object), store just the relations:
student.teachers.applyChangesToDb();

student.teachers.removeAt(0)
// Simply put the student again:
// boxStore.boxFor(Student::class.java).put(student)
// Or more efficient (than writing the thole object), store just the relations:
student.teachers.applyChangesToDb()

student.teachers.removeAt(0)
// Simply put the student again:
// store.box<Student>().put(student);
// Or more efficient (than writing the thole object), store just the relations:
student.teachers.applyToDb();

Access Many-To-Many in the reverse direction

Following the above example, you might want an easy way to find out what students a teacher has. Instead of having to perform a query, you can just add a to-many relation to the teacher and annotate it with the @Backlink annotation:

// Teacher.java
@Entity
public class Teacher{
    
    @Id public long id;
    
    // Backed by the to-many relation in Student:
    @Backlink(to = "teachers")
    public ToMany<Student> students;
    
}
​
// Student.java
@Entity
public class Student{
    
    @Id public long id;
    
    public ToMany<Teacher> teachers;
    
}

@Entity
data class Teacher(
        @Id var id: Long = 0
) {
    // Backed by the to-many relation in Student:
    @Backlink(to = "teachers")
    lateinit var students: ToMany<Student>
}
​
@Entity
data class Student(
        @Id var id: Long = 0
) {
    lateinit var teachers: ToMany<Teacher>
}

@Entity()
class Teacher{
  int id;
  
  // Backed by the to-many relation in Student:
  @Backlink()
  final students = ToMany<Student>();
}
​
@Entity()
public class Student{
  int id;
  
  final teachers = ToMany<Teacher>();
}

Updating Relations

The ToOne and ToMany relation classes assist you to persist the relation state. They keep track of changes and apply them to the database once you put the Object containing them. ObjectBox supports relation updates for new (not yet persisted; ID == 0) and existing (persisted before; ID != 0) Objects.

For convenience, ToOne and ToMany will put Objects added to them if they do not yet exist (ID == 0). If they already exist (their ID != 0, or you are using @Id(assignable = true)), only the relation will be created or destroyed (internally the Object ID is added to or removed from the relation). In that case, to put changes to the properties of related Objects use their specific Box instead:

// update a related entity using its box
Order orderToUpdate = customer.orders.get(0);
orderToUpdate.text = "Revised description";
// DOES NOT WORK
// boxStore.boxFor(Customer.class).put(customer);
// WORKS
boxStore.boxFor(Order.class).put(orderToUpdate);

// update a related entity using its box
Order orderToUpdate = customer.orders.getAt(0);
orderToUpdate.text = "Revised description";
// DOES NOT WORK
// boxStore.boxFor(Customer::class.java).put(customer);
// WORKS
boxStore.boxFor(Order::class.java).put(orderToUpdate);

// update a related entity using its box
Order orderToUpdate = customer.orders[0];
orderToUpdate.text = 'Revised description';
// DOES NOT WORK - the change to the order is not saved
// store.box<Customer>().put(customer);
// WORKS
store.box<Order>().put(orderToUpdate);

Updating ToOne

The ToOne class offers the following methods to update the relation:

• setTarget(target) makes the given (new or existing) Object the new relation target; pass null to clear the relation.

• setTargetId(targetId) sets the relation target based on the given ID of an existing Object; pass 0 (zero) to clear the relation.

• Java/Kotlin only: setAndPutTarget(target) makes the given (new or existing) Object the new relation target and puts the owning, source Object and if needed the target Object.

// Option 1: set target and put.
order.customer.setTarget(customer);
// Or set target via Object ID: 
// order.customer.setCustomerId(customer.getId());
orderBox.put(order);
​
// Option 2: combined set target and put.
order.customer.setAndPutTarget(customer);

// Option 1: set target and put.
order.customer.setTarget(customer)
// Or set target via Object ID: 
// order.customer.setCustomerId(customer.getId())
orderBox.put(order)
​
// Option 2: combined set target and put.
order.customer.setAndPutTarget(customer)

Order order = new Order();
orderBox.attach(order);
order.customer.setAndPutTarget(customer);

Customer customer = new Customer();
// If ID is self-assigned, put target Object first
customer.id = 12;
customerBox.put(customer);
// Then can safely set as target
order.customer.setTarget(customer);
// Or set target via Object ID
// order.customer.setCustomerId(customer.getId());
orderBox.put(order);

Updating ToMany

The ToMany relation class is based on a standard List with added change tracking for Objects. As mentioned above, it will put new Objects (ID == 0) that are added to it once the Object owning it is put. And when removing Objects from it, just the relation is cleared, the Objects are not removed from their Box.

See the documentation on One-to-Many and Many-to-Many above for details.

If the owning, source Object uses @Id(assignable = true) attach its Box before modifying its ToMany:

// If source has self-assigned ID attach Box first
customer.id = 12;
customerBox.attach(customer);
// Then can safely modify ToMany
customer.orders.add(order);
customerBox.put(customer);

If the target Object, like Order above, is using self-assigned IDs put the target Objects before adding them to the ToMany relation:

// If ID is self-assigned put target Object first
order.id = 42;
orderBox.put(order);
// Then can safely add target Object to ToMany
customer.orders.add(order);
customerBox.put(customer);

The above steps are required because, when putting the Object owning the ToMany only the relation is updated. This is because ObjectBox only puts target Objects with an ID of 0. See the documentation about IDs for the background information.

Example: Extending the Model with an Address

A typical extension to the customer/order example we have started earlier would be to add an Address type. While you could model street, ZIP and so on directly into Customer, it usually makes sense to normalize that out into a separate entity. And when you think about the usual shopping sites, they all let you have multiple addresses...

So, typically, this is how you would model that:

• an address is bound to a customer; add a ToOne<Customer> customer to Address

• an order is shipped to one address, and might have a diverging billing address optionally; add ToOne<Address> shippingAddress and ToOne<Address> billingAddress to Order

• For each ToOne you could have a matching ToMany on the other side of the relation (backlink)

Example: Modelling Tree Relations

You can model a tree relation with a to-one and a to-many relation pointing to itself:

@Entity
public class TreeNode {
    @Id long id;
    
    ToOne<TreeNode> parent;
    
    @Backlink
    ToMany<TreeNode> children;
}

@Entity
data class TreeNode(
        @Id var id: Long = 0
) {
    lateinit var parent: ToOne<TreeNode>
    
    @Backlink
    lateinit var children: ToMany<TreeNode>
}

This lets you navigate a tree nodes parent and children: