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Whenever you are doing changes to your entities, you're required to also migrate the old data.

First, update your entity. Next, Increase the database version. Define a Migration which specifies a startVersion, an endVersion and a function that executes SQL to migrate the data. At last, use addMigrations() on the obtained database builder to add migrations. Don't forget to trigger the code generator again, to create the code for handling the new entity.

// update entity with new 'nickname' field
@Entity(tableName: 'person')
class Person {
  @PrimaryKey(autoGenerate: true)
  final int id;

  @ColumnInfo(name: 'custom_name')
  final String name;

  final String nickname;

  Person(,, this.nickname);

// bump up database version
@Database(version: 2)
abstract class AppDatabase extends FloorDatabase {
  PersonDao get personDao;

// create migration
final migration1to2 = Migration(1, 2, (database) async {
  await database.execute('ALTER TABLE person ADD COLUMN nickname TEXT');

final database = await $FloorAppDatabase