How to define metaclass for a class that extends from sqlalchemy declarative base in python

I use: Python 2.6 and sqlalchemy 0.6.1

This is what I am trying to do:

 from sqlalchemy.types import (
 Integer,
 String,
 Boolean
 )
 from sqlalchemy.ext.declarative import declarative_base

 Base = declarative_base()

 class SampleMeta(type):
 def __new__(cls, name, bases, attrs):
 attrs.update({ 'id': Column('Id', Integer, primary_key=True),
 'name': Column('Name', String),
 'description': Column('Description', String),
 'is_active': Column('IsActive', Boolean)
 })
 return super(SampleMeta, cls).__new__(cls, name, bases, attrs)

 class Sample(Base):
 __tablename__ = 'Sample'
 __table_args__ = {'useexisting': True}
 __metaclass__ = SampleMeta

 def __init__(self, id, name, description, is_active):
 self.id = id
 self.name = name
 self.description = description
 self.is_active = is_active

 def __repr__(self):
 return "" % (self.id, self.name, self.description, self.isactive)

And the error I am getting is this:

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

Now, if I do the same thing above by using

 class Sample(object)

instead of

 class Sample(Base)

it works absolutely fine.

I need to update the attributes of the class dynamically. So, I will be using dynamic attribute and column names. And I need the above piece code to work in order to be able to get there.

1 Answer

I'm not an sqlalchemy user, but I'd try deriving your metaclass from Base's metaclass:

BaseMeta = type(Base)

class SampleMeta(BaseMeta):
 ...

class Sample(Base):
 __metaclass__ = SampleMeta
 ...

answer Jul 3, 2013 by anonymous

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