Primary Keys - Kotlin SDK

Define a primary key for an object type with the @PrimaryKey annotation:

class Lizard : RealmObject {
    @PrimaryKey
    val _id: ObjectId = ObjectId()
}

Realm Database treats fields marked with the @PrimaryKey annotation as primary keys for their corresponding object schema. Primary keys are subject to the following limitations:

• You can define only one primary key per object schema.

• Primary key values must be unique across all instances of an object in a realm. Attempting to insert a duplicate primary key value results in an error.

• Primary key values are immutable. To change the primary key value of an object, you must delete the original object and insert a new object with a different primary key value.

• You cannot change the primary key field for an object type after adding any object of that type to a realm.

You can create a primary key with any of the following types:

• String

• Byte

• Char

• Short

• Int

• Long

• ObjectId

• RealmUUID

Optional fields can contain a value of null as a primary key value, but only for one object of a particular type, since each primary key value must be unique. Attempting to insert an object with an existing primary key into a realm results in an error.

Realm Database automatically indexes primary key fields, which allows you to efficiently read and modify objects based on their primary key.