在App更新的过程中，不免需要升级数据库，重建或修改表结构，但是GreenDao并没有提供相应的数据库升级方法，观察GreenDao生成的DaoMaster类，可以找到解决办法。

GreenDao中数据库版本不可修改，在编译生成的DaoMaster类中，可以看到，SCHEMA_VERSION固定为1。

public class DaoMaster extends AbstractDaoMaster {
    public static final int SCHEMA_VERSION = 1;

    ...

    public static abstract class OpenHelper extends DatabaseOpenHelper {
        public OpenHelper(Context context, String name) {
            super(context, name, SCHEMA_VERSION);
        }

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

        @Override
        public void onCreate(Database db) {
            Log.i("greenDAO", "Creating tables for schema version " + SCHEMA_VERSION);
            createAllTables(db, false);
        }
    }

    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);
        }
    }

}

一般GreenDao初始化为:

    DaoMaster.DevOpenHelper helper = new DaoMaster.DevOpenHelper(this, ENCRYPTED ? "fpc-db-encrypted" : "fpc-db");
    db = ENCRYPTED ? helper.getEncryptedWritableDb("super-secret") : helper.getWritableDb();
    daoSession = new DaoMaster(db).newSession();

如果使用自带的DevOpenHelper，那么生成的数据库版本固定为SCHEMA_VERSION = 1，数据库永远不会升级。

要想升级数据库版本从而触发onUpgrade方法，需要自定义一个OpenHelper，继承org.greenrobot.greendao.database.DatabaseOpenHelper，代码如下:

...
import org.greenrobot.greendao.database.DatabaseOpenHelper;

public class GreenOpenHelper extends DatabaseOpenHelper {

    public static int DB_VERSION = BuildConfig.VERSION_CODE;

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

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

    @Override
    public void onCreate(Database db) {
        Log.i("greenDAO", "Creating tables for schema version " + DB_VERSION);
        DaoMaster.createAllTables(db, false);
    }

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

注意， DatabaseOpenHelper继承android.database.sqlite.SQLiteOpenHelper，需要将数据库版本写入SQLiteOpenHelper的构造器才能生效

我们使用GreenOpenHelper替代DaoMaster.DevOpenHelper，初始化代码现在变为:

    GreenOpenHelper helper = new GreenOpenHelper(this, ENCRYPTED ? "fpc-db-encrypted" : "fpc-db");
    db = ENCRYPTED ? helper.getEncryptedWritableDb("super-secret") : helper.getWritableDb();
    daoSession = new DaoMaster(db).newSession();

现在我们可以用BuildConfig.VERSION_CODE来控制数据库版本，需要升级数据库时在build.gradle(app模块)修改versionCode就可以了。