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Embedded Objects

Overview

An embedded object is a special type of Realm object that models complex data about a specific object. Embedded objects are similar to relationships, but they provide additional constraints and map more naturally to the denormalized MongoDB document model.

Realm enforces unique ownership constraints that treat each embedded object as nested data inside of a single, specific parent object. An embedded object inherits the lifecycle of its parent object and cannot exist as an independent Realm object. Realm automatically deletes embedded objects if their parent object is deleted or when overwritten by a new embedded object instance.

Realm Uses Cascading Deletes for Embedded Objects

When you delete a Realm object, Realm automatically deletes any embedded objects referenced by that object. Any objects that your application must persist after the deletion of their parent object should use relationships instead.

Embedded Object Data Models

You can define embedded object types using either Realm object models or a server-side document schema. Embedded object types are reusable and composable. You can use the same embedded object type in multiple parent object types and you can embed objects inside of other embedded objects.

Important

Embedded objects cannot have a primary key.

Realm Object Models

To specify that a Realm object model defines an embedded object, derive the embedded object class from RealmObject and set the embedded property of the RealmClass annotation to true. You can reference an embedded object type from parent object types in the same way as you would define a relationship:

// Define an embedded object
@RealmClass(embedded = true)
open class Address(
    var street: String? = null,
    var city: String? = null,
    var country: String? = null,
    var postalCode: String? = null
): RealmObject() {}

// Define an object containing one embedded object
open class Contact(_name: String = "", _address: Address? = null) : RealmObject() {
    @PrimaryKey var _id: ObjectId = ObjectId()
    var name: String = _name
    
    // Embed a single object.
    // Embedded object properties must be marked optional
    var address: Address? = _address
}

// Define an object containing an array of embedded objects
open class Business(_name: String = "", _addresses: RealmList<Address> = RealmList()) : RealmObject() {
    @PrimaryKey var _id: ObjectId = ObjectId()
    var name: String = _name
    
    // Embed an array of objects
    var addresses: RealmList<Address> = _addresses
}
// Define an embedded object
@RealmClass(embedded = true)
public class Address extends RealmObject {
    String street;
    String city;
    String country;
    String postalCode;

    public Address(String street, String city, String country, String postalCode) {
        this.street = street;
        this.city = city;
        this.country = country;
        this.postalCode = postalCode;
    }

    public Address() {}
}

// Define an object containing one embedded object
public class Contact extends RealmObject {
    @PrimaryKey
    private ObjectId _id = new ObjectId();
    String name = "";

    // Embed a single object.
    // Embedded object properties must be marked optional
    Address address;

    public Contact(String name, Address address) {
        this.name = name;
        this.address = address;
    }

    public Contact() {}
}

// Define an object containing an array of embedded objects
public class Business extends RealmObject {
    @PrimaryKey
    private ObjectId _id = new ObjectId();
    String name = "";
    
    // Embed an array of objects
    RealmList<Address> addresses = new RealmList<Address>();

    public Business(String name, RealmList<Address> addresses) {
        this.name = name;
        this.addresses = addresses;
    }

    public Business() {}
}

JSON Schema

Unlike regular Realm objects, which map to their own MongoDB collection, embedded objects map to embedded documents in the parent type’s document schema:

{
  "title": "Contact",
  "bsonType": "object",
  "required": ["_id", "name", "address"],
  "properties": {
    "_id": "objectId",
    "name": "string",
    "address": {
      "title": "Address",
      "bsonType": "object",
      "properties": {
        "street": "string",
        "city": "string",
        "country": "string",
        "postalCode": "string"
      }
    }
  }
}
{
  "title": "Business",
  "bsonType": "object",
  "required": ["_id", "name", "addresses"],
  "properties": {
    "_id": "objectId",
    "name": "string",
    "addresses": {
      "bsonType": "array",
      "items": {
        "title": "Address",
        "bsonType": "object",
        "properties": {
          "street": "string",
          "city": "string",
          "country": "string",
          "postalCode": "string"
        }
      }
    }
  }
}

Read and Write Embedded Objects

Create an Embedded Object

To create an embedded object, assign an instance of the embedded object to a parent object’s property:

val user: User? = app.currentUser()
val partitionValue: String = "<partition>" // replace this with a partition key
val config = SyncConfiguration.Builder(user!!, partitionValue)
    .build()
val realm: Realm = Realm.getInstance(config)

val address = Address("123 Fake St.", "Springfield", "USA", "90710")
val contact = Contact("Nick Riviera", address)

realm.executeTransaction { transactionRealm ->
    transactionRealm.insert(contact)
}

realm.close()
User user = app.currentUser();
String partitionValue = "My Project";
SyncConfiguration config = new SyncConfiguration.Builder(user, partitionValue)
        .build();

Realm realm = Realm.getInstance(config);

Address address = new Address("123 Fake St.", "Springfield", "USA", "90710");
Contact contact = new Contact("Nick Riviera", address);

realm.executeTransaction(transactionRealm -> {
    transactionRealm.insert(contact);
});

realm.close();

Update an Embedded Object Property

To update a property in an embedded object, modify the property in a write transaction:

// assumes that at least one contact already exists in this partition
val result = realm.where<Contact>().findFirst()!!

realm.executeTransaction { transactionRealm ->
    result.address?.street = "Hollywood Upstairs Medical College"
    result.address?.city = "Los Angeles"
    result.address?.postalCode = "90210"
    Log.v("EXAMPLE", "Updated contact: ${result.name}")
}

realm.close()
// assumes that at least one contact already exists in this partition 
Contact resultContact = realm.where(Contact.class).findFirst();

realm.executeTransaction(transactionRealm -> {
    resultContact.address.street = "Hollywood Upstairs Medical College";
    resultContact.address.city = "Los Angeles";
    resultContact.address.postalCode = "90210";
    Log.v("EXAMPLE", "Updated contact: " + resultContact);
});

realm.close();

Overwrite an Embedded Object

To overwrite an embedded object, reassign the embedded object property of a party to a new instance in a write transaction:

// assumes that at least one contact already exists in this partition
val oldContact = realm.where<Contact>().findFirst()!!

realm.executeTransaction { transactionRealm ->
    val newAddress = Address(
        "Hollywood Upstairs Medical College",
        "Los Angeles",
        "USA",
        "90210")
    oldContact.address = newAddress
    Log.v("EXAMPLE", "Updated contact: $oldContact")
}

realm.close()
// assumes that at least one contact already exists in this partition 
Contact oldContact = realm.where(Contact.class).findFirst();

realm.executeTransaction(transactionRealm -> {
    Address newAddress = new Address(
        "Hollywood Upstairs Medical College",
        "Los Angeles",
        "USA"
        "90210"
        );
    oldContact.address = newAddress;
    Log.v("EXAMPLE", "Replaced contact: " + oldContact);
});

realm.close();

Query a Collection on Embedded Object Properties

Use dot notation to filter or sort a collection of objects based on an embedded object property value:

Note

It is not possible to query embedded objects directly. Instead, access embedded objects through a query for the parent object type.

val losAngelesContacts = realm.where<Contact>()
    .equalTo("address.city", "Los Angeles")
    .sort("address.street").findAll()
Log.v("EXAMPLE", "Los Angeles Contacts: $losAngelesContacts")
RealmResults<Contact> losAngelesContacts = realm.where(Contact.class)
        .equalTo("address.city", "Los Angeles")
        .sort("address.street").findAll();
Log.v("EXAMPLE", "Los Angeles contacts: " + losAngelesContacts);