唯一约束在android的房间数据库中不起作用

@Entity(tableName = "users", indices = {@Index(value = {"email"}, unique = true)})
public class User {
    @PrimaryKey(autoGenerate = true)
    public int id;

    @ColumnInfo(name = "email")
    public String email;
}

RoomDatabase.Callback callback = new RoomDatabase.Callback() {
    @Override
    public void onCreate(@NonNull SupportSQLiteDatabase db) {
        super.onCreate(db);
        // 执行迁移代码
    }
};

MyDatabase db = Room.databaseBuilder(context, MyDatabase.class, "my-database")
        .addCallback(callback)
        .build();

@Override
public void onCreate(@NonNull SupportSQLiteDatabase db) {
    super.onCreate(db);
    db.execSQL("CREATE UNIQUE INDEX unique_user_email ON users(email);");
}