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

Peter Otten __peter__ at web.de
Wed Jul 3 07:37:42 CEST 2013


Ven wrote:

> 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.

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
    ...






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