使用场景

版本升级后，增加表、删除表、增加字段、删减字段等。为了保留原始数据，需要将数据进行迁移。使用GreenDao数据库的升级操作该如何进行呢？

GreenDao的简单实现可以参考：http://www.jianshu.com/p/4986100eff90

思路

• 创建临时表TMP_，将原先表数据迁移过去
• 删除所有原表
• 创建新表
• 将TMP_表数据复制到新表，最后将TMP_表删除

实现

• 添加字段并编译

private static void addUser(Schema schema) {
        Entity user = schema.addEntity("UserInfo");
        user.addIdProperty();
        user.addStringProperty("name").notNull();
        user.addStringProperty("idNumber").notNull();
        user.addStringProperty("department").notNull();
        user.addStringProperty("place");// 新增字段
        user.addStringProperty("project");
        user.addStringProperty("updateTime");
        user.addBooleanProperty("isSelected");
    }

编译后，会产生新的UserInfo、UserInfoDao、DaoMaster、DaoSession。我们将新产生的类，替换原先的类。

• 实现MigrationHelper

public class MigrationHelper {

    private static final String CONVERSION_CLASS_NOT_FOUND_EXCEPTION = "MIGRATION HELPER - CLASS DOESN'T MATCH WITH THE CURRENT PARAMETERS";
    private static MigrationHelper instance;

    static MigrationHelper getInstance() {
        if(instance == null) {
            instance = new MigrationHelper();
        }
        return instance;
    }

    @SafeVarargs
    final void migrate(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {
        generateTempTables(db, daoClasses);
        DaoMaster.dropAllTables(db, true);
        DaoMaster.createAllTables(db, false);
        restoreData(db, daoClasses);
    }

    @SafeVarargs
    private final void generateTempTables(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {
        for (Class<? extends AbstractDao<?, ?>> daoClass : daoClasses) {
            DaoConfig daoConfig = new DaoConfig(db, daoClass);

            String divider = "";
            String tableName = daoConfig.tablename;
            String tempTableName = daoConfig.tablename.concat("_TEMP");
            ArrayList<String> properties = new ArrayList<>();

            StringBuilder createTableStringBuilder = new StringBuilder();

            createTableStringBuilder.append("CREATE TABLE ").append(tempTableName).append(" (");

            for (int j = 0; j < daoConfig.properties.length; j++) {
                String columnName = daoConfig.properties[j].columnName;

                if (getColumns(db, tableName).contains(columnName)) {
                    properties.add(columnName);

                    String type = null;

                    try {
                        type = getTypeByClass(daoConfig.properties[j].type);
                    } catch (Exception exception) {
//                        Crashlytics.logException(exception);
                    }

                    createTableStringBuilder.append(divider).append(columnName).append(" ").append(type);

                    if (daoConfig.properties[j].primaryKey) {
                        createTableStringBuilder.append(" PRIMARY KEY");
                    }

                    divider = ",";
                }
            }
            createTableStringBuilder.append(");");

            db.execSQL(createTableStringBuilder.toString());

            String insertTableStringBuilder = "INSERT INTO " + tempTableName + " (" +
                    TextUtils.join(",", properties) +
                    ") SELECT " +
                    TextUtils.join(",", properties) +
                    " FROM " + tableName + ";";

            db.execSQL(insertTableStringBuilder);
        }
    }

    @SafeVarargs
    private final void restoreData(Database db, Class<? extends AbstractDao<?, ?>>... daoClasses) {
        for (Class<? extends AbstractDao<?, ?>> daoClass : daoClasses) {
            DaoConfig daoConfig = new DaoConfig(db, daoClass);

            String tableName = daoConfig.tablename;
            String tempTableName = daoConfig.tablename.concat("_TEMP");
            List<String> properties = new ArrayList<>();

            for (int j = 0; j < daoConfig.properties.length; j++) {
                String columnName = daoConfig.properties[j].columnName;

                if (getColumns(db, tempTableName).contains(columnName)) {
                    properties.add(columnName);
                }
            }

            String insertTableStringBuilder = "INSERT INTO " + tableName + " (" +
                    TextUtils.join(",", properties) +
                    ") SELECT " +
                    TextUtils.join(",", properties) +
                    " FROM " + tempTableName + ";";

            db.execSQL(insertTableStringBuilder);
            db.execSQL("DROP TABLE " + tempTableName);
        }
    }

    private String getTypeByClass(Class<?> type) throws Exception {
        if(type.equals(String.class)) {
            return "TEXT";
        }
        if(type.equals(Long.class) || type.equals(Integer.class) || type.equals(long.class)) {
            return "INTEGER";
        }
        if(type.equals(Boolean.class)) {
            return "BOOLEAN";
        }

        //        Crashlytics.logException(exception);
        throw new Exception(CONVERSION_CLASS_NOT_FOUND_EXCEPTION.concat(" - Class: ").concat(type.toString()));
    }

    private static List<String> getColumns(Database db, String tableName) {
        List<String> columns = new ArrayList<>();
        Cursor cursor = null;
        try {
            cursor = db.rawQuery("SELECT * FROM " + tableName + " limit 1", null);
            if (cursor != null) {
                columns = new ArrayList<>(Arrays.asList(cursor.getColumnNames()));
            }
        } catch (Exception e) {
            Log.v(tableName, e.getMessage(), e);
            e.printStackTrace();
        } finally {
            if (cursor != null)
                cursor.close();
        }
        return columns;
    }
}

• 修改DaoMaster
  修改数据库版本： public static final int SCHEMA_VERSION = 4;
  在DevOpenHelper的onUpgrade方法中，修改升级操作。
  在升级时，需将所有的Dao类加至migrate方法中。示例只有一个Dao类。

 @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) {
                // 升级、数据库迁移操作
                MigrationHelper.getInstance().migrate(db, UserInfoDao.class);
            }else {
                // 默认操作
                dropAllTables(db, true);
                onCreate(db);
            }
        }