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Model Data - Java SDK

docs/guides/model-data.md

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Model Data - Java SDK

An object schema is a configuration object that defines the fields and relationships of a Realm object type. Android Realm applications define object schemas with Java or Kotlin classes using Realm Schemas.

Object schemas specify constraints on object fields such as the data type of each field, whether a field is required, and default field values. Schemas can also define relationships between object types in a realm.

Modifying your application's Realm Schema requires you to migrate data from older versions of your Realm Schema to the new version.

Realm Apps

Every App has a Realm Schema composed of a list of object schemas for each type of object that the realms in that application may contain.

Realm guarantees that all objects in a realm conform to the schema for their object type and validates objects whenever they're created, modified, or deleted.

Relationships

You can model one-to-one relationships in realm with RealmObject fields. You can model one-to-many and many-to-one relationships RealmList fields. Inverse relationships are the opposite end of a one-to-many or many-to-one relationship. You can make inverse relationships traversable with the @LinkingObjects annotation on a RealmResults field. In an instance of a RealmObject, inverse relationship fields contain the set of Realm objects that point to that object instance through the described relationship. You can find the same set of Realm objects with a manual query, but the inverse relationship field reduces boilerplate query code and capacity for error.

Realm Objects

Unlike normal Java objects, which contain their own data, a Realm object doesn't contain data. Instead, Realm objects read and write properties directly to Realm.

Instances of Realm objects can be either managed or unmanaged.

  • Managed objects are: persisted in Realmalways up to datethread-confinedgenerally more lightweight than the unmanaged version, as they take up less space on the Java heap.
  • Unmanaged objects are just like ordinary Java objects, since they are not persisted and never update automatically. You can move unmanaged objects freely across threads.

You can convert between the two states using realm.copyToRealm() and realm.copyFromRealm().

RealmProxy

The RealmProxy classes are the Realm SDK's way of ensuring that Realm objects don't contain any data themselves. Instead, each class's RealmProxy accesses data directly in the database.

For every model class in your project, the Realm annotation processor generates a corresponding RealmProxy class. This class extends your model class and is returned when you call Realm.createObject(). In your code, this object works just like your model class.

Realm Object Limitations

Realm objects:

  • cannot contain fields that use the final or volatile modifiers (except for inverse relationship fields).
  • cannot extend any object other than RealmObject.
  • must contain an empty constructor (if your class does not include any constructor, the automatically generated empty constructor will suffice)

Naming limitations:

  • Class names cannot exceed 57 characters.
  • Class names must be unique within realm modules
  • Field names cannot exceed 63 characters.

Size limitations:

  • String or byte[] fields cannot exceed 16 MB.

Usage limitations:

  • Because Realm objects are live and can change at any time, their hashCode() value can change over time. As a result, you should not use RealmObject instances as a key in any map or set.

Incremental Builds

The bytecode transformer used by Realm supports incremental builds, but your application requires a full rebuild when adding or removing the following from a Realm object field:

  • an @Ignore annotation
  • the static keyword
  • the transient keyword

You can perform a full rebuild with Build > Clean Project and Build > Rebuild Project in these cases.

Schema Version

A schema version identifies the state of a Realm Schema at some point in time. Realm tracks the schema version of each realm and uses it to map the objects in each realm to the correct schema.

Schema versions are integers that you may include in the realm configuration when you open a realm. If a client application does not specify a version number when it opens a realm then the realm defaults to version 0.

Important: Migrations must update a realm to a higher schema version. Realm throws an error if a client application opens a realm with a schema version that is lower than the realm's current version or if the specified schema version is the same as the realm's current version but includes different object schemas.

Migrations

A local migration is a migration for a realm with another realm. Local migrations have access to the existing Realm Schema, version, and objects and define logic that incrementally updates the realm to its new schema version. To perform a local migration you must specify a new schema version that is higher than the current version and provide a migration function when you open the out-of-date realm.

With the SDK, you can update underlying data to reflect schema changes using manual migrations. During such a manual migration, you can define new and deleted properties when they are added or removed from your schema. The editable schema exposed via a DynamicRealm provides convenience functions for renaming fields. This gives you full control over the behavior of your data during complex schema migrations.

Tip: During development of an application, RealmObject classes can change frequently. You can use Realm.deleteRealm()to delete the database file and eliminate the need to write a full migration for testing data.