How do you store custom data types (such as dates or lists) in Room using TypeConverters?

Short answer

By defining functions annotated with @TypeConverter to convert the custom type to a primitive type SQLite understands (such as text or a number) and reverse it on read.

Explanation

SQLite supports limited types (NULL, INTEGER, REAL, TEXT, BLOB). If you have a field of type Date or List<String> in the entity, you create an intermediary class that converts the date to a Long (timestamp) and back, or converts the list to JSON text, then register the class in the database through @TypeConverters.

Code example

class Converters {
    @TypeConverter
    fun fromTimestamp(value: Long?): Date? = value?.let { Date(it) }

    @TypeConverter
    fun dateToTimestamp(date: Date?): Long? = date?.time
}

@Database(entities = [Note::class], version = 1)
@TypeConverters(Converters::class)
abstract class AppDatabase : RoomDatabase()

Common mistakes

• Using a TypeConverter to convert whole objects tied by one-to-many relationships instead of creating native tables and relations in the database.

Follow-up questions

• What are the three basic components of the Room library and what is the role of each?

Sources

• Android Developers — Use type converters in Room