metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

解决：

因为theme,product,theme_product三张表相互引用，所以就写成了死循环。怎么改都不对。

最终做法是把在theme中写ManytoManyFiled,theme和product多对多关系成立，自动创建了theme_product表，但是theme_product表没有注册进xadmin。使用python manage.py inspectdb检查出theme_product表的写法，把它写到models里面，就也注册进xadmin了