Dart APIunself_local_databaseFieldCompanionextra extra property Null safety Value<String> extra final Implementation final Value<String> extra; unself_local_database unself_local_database FieldCompanion extra property FieldCompanion class Constructors FieldCompanion insert Properties collectionId created extra hashCode id name required runtimeType system type unique updated Methods copyWith noSuchMethod toColumns toString Operators operator == Static methods custom