flask-sqlalchemy分表解决方案

大型系统、海量数据肯定涉及到分库分表这些提高效率的手段。由于sqlalchemy的orm思想是一张表对应一个对象，那么当我们有N张相同结构只是表名有区别的分表，sqlalchemy orm怎样处理呢。比如有如下表：

CREATE TABLE `goods_desc_0` (
  `goods_id` bigint(20) unsigned NOT NULL DEFAULT '0' COMMENT '商品id',
  `goods_desc` text NOT NULL COMMENT '商品详细描述',
  PRIMARY KEY (`goods_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=COMPACT COMMENT='商品信息详情'

CREATE TABLE `goods_desc_1` (
  `goods_id` bigint(20) unsigned NOT NULL DEFAULT '0' COMMENT '商品id',
  `goods_desc` text NOT NULL COMMENT '商品详细描述',
  PRIMARY KEY (`goods_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=COMPACT COMMENT='商品信息详情'

class GoodsDesc(db.Model):
    __tablename__ = 'goods_desc'

    goods_id = db.Column(db.Integer, primary_key=True)
    goods_desc = db.Column(db.Text, default=None)

    def __str__(self):
        return "GoodsDesc => { 
goods_id:%d, goods_desc:'%s'}" % (
self.goods_id, self.goods_desc)

    __repr__ = __str__

# 代码示例
goods_id = 101
table_index = goods_id%2
table_name = 'goods_desc_%d' % table_index
GoodsDesc.__table__.name = table_name
gd = GoodsDesc.query.filter(GoodsDesc.goods_id == goods_id).first()

# 这样写虽然也能工作，但是是非常危险的，因为GoodsDesc.__table__是静态全局变量，
# 而且不是web程序request级别的，是app context的，非常不安全。

经过N次Google，终于找到解决方案，而且代码写起来也很优雅。代码如下：

class GoodsDesc(object):

    _mapper = {}

    @staticmethod
    def model(goods_id):
        table_index = goods_id%100
        class_name = 'GoodsDesc_%d' % table_index

        ModelClass = GoodsDesc._mapper.get(class_name, None)
        if ModelClass is None:
            ModelClass = type(class_name, (db.Model,), {
                '__module__' : __name__,
                '__name__' : class_name,
                '__tablename__' : 'goods_desc_%d' % table_index,

                'goods_id' : db.Column(db.Integer, primary_key=True),
                'goods_desc' : db.Column(db.Text, default=None),
            })
            GoodsDesc._mapper[class_name] = ModelClass

        cls = ModelClass()
        cls.goods_id = goods_id
        return cls

# 外部代码调用如例如下：
# -----------------------

# 新增插入
gdm = GoodsDesc.model(goods_id)
gdm.goods_desc = 'desc'
db.session.add(gd)

# 查询
gdm = GoodsDesc.model(goods_id)
gd = gdm.query.filter_by(goods_id=goods_id).first()