默认的 DevOpenHelper 不能用，数据库版本号改变时每次都是删除重建

 /** WARNING: Drops all table on Upgrade! Use only during development. */
    public static class DevOpenHelper extends OpenHelper {
        public DevOpenHelper(Context context, String name) {
            super(context, name);
        }

        public DevOpenHelper(Context context, String name, CursorFactory factory) {
            super(context, name, factory);
        }

        @Override
        public void onUpgrade(Database db, int oldVersion, int newVersion) {
            Log.i("greenDAO", "Upgrading schema from version " + oldVersion + " to " + newVersion + " by dropping all tables");
            dropAllTables(db, true);
            onCreate(db);
        }
    }

其实之前直接继承 SQLiteOpenHelper 怎么升级现在就怎么升级，因为 greendao 的 OpenHelper 也是继承了 SQLiteOpenHelper 简单扩展了一下而已
所以我们再自己写一个 OpenHelper 重写 onUpgrade 就可以了

 public class SQLiteOpenHelper extends DaoMaster.OpenHelper {
    private static HashMap<Integer, Migration> ALL_MIGRATIONS = new HashMap<>();

    static {
        // 每一个数据库版本对应一个这个版本的升级脚本
        ALL_MIGRATIONS.put(2, new V2Migration());
    }

    public SQLiteOpenHelper(Context context, String name) {
        super(context, name);
    }

    public SQLiteOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory) {
        super(context, name, factory);
    }

    @Override
    public void onUpgrade(Database db, int oldVersion, int newVersion) {
        Log.d("db", "oldVersion " + oldVersion + " newVersion " + newVersion);
        Set versionSet = ALL_MIGRATIONS.keySet();
        // 从当前版本的下一个版本到最新版本的所有的迁移脚本都要跑一遍
        // 如当前数据库版本是 2 ，现在要装一个版本号是 5 的 app， 则 3 4 5 的迁移脚本都要跑一遍
        for (int v = oldVersion + 1; v <= newVersion; v++) {
            if (versionSet.contains(v)) {
                ALL_MIGRATIONS.get(v).migrate(db);
            }
        }
    }

    interface Migration {
        void migrate(Database db);
    }

    static class V2Migration implements Migration {
        @Override
        public void migrate(Database db) {
            db.execSQL("alter table " + NoteDao.TABLENAME + " add column " + NoteDao.Properties.Age.columnName + " int");
        }
    }
}

使用

ProOpenHelper helper = new ProOpenHelper(this, "hj-db");
Database db = helper.getWritableDb();
daoSession = new DaoMaster(db).newSession();