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

Ven praveen.venkata at gmail.com
Wed Jul 3 06:12:09 CEST 2013


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 "<(%d, '%s', '%s', %r)>" % (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.

**Please help** 



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