Embedded

Marks a field of an Entity or POJO to allow nested fields (i.e. fields of the annotated field's class) to be referenced directly in the SQL queries.

If the container is an Entity, these sub fields will be columns in the Entity's database table.

For example, if you have 2 classes:

data class Coordinates (
  val latitude: Double,
  val longitude: Double
)

data class Address (
  val street: String,
  @Embedded
  val coordinates: Coordinates
)

Room will consider latitude and longitude as if they are fields of the Address class when mapping an SQLite row to Address.

So if you have a query that returns street, latitude, longitude, Room will properly construct an Address class.

If the Address class is annotated with Entity, its database table will have 3 columns: street, latitude and longitude.

If there is a name conflict with the fields of the sub object and the owner object, you can specify a prefix for the fields of the sub object. Note that prefix is always applied to sub fields even if they have a ColumnInfo with a specific name.

If sub fields of an embedded field has PrimaryKey annotation, they will not be considered as primary keys in the owner Entity.

When an embedded field is read, if all fields of the embedded field (and its sub fields) are null in the android.database.Cursor, it is set to null. Otherwise, it is constructed.

Note that even if you have TypeConverters that convert a null column into a non-null value, if all columns of the embedded field in the android.database.Cursor are null, the TypeConverter will never be called and the embedded field will not be constructed.

You can override this behavior by annotating the embedded field with androidx.annotation.NonNull.

Summary

Embedded(prefix: String = "")

val prefix: String

@Embedded(prefix = "loc_")
val coordinates: Coordinates