On Sep 22, 2010, at 4:30 AM, Yap Sok Ann wrote:
This is related to topic need 0.6_beta2-compat declarative meta
http://groups.google.com/group/sqlalchemy/browse_thread/thread/ae7cb9d2ab0b9cca
Prior to version 0.6, I use the following code to automatically add a
primary key if the table doesn't have one defined:
from sqlalchemy.ext.declarative import declarative_base,
DeclarativeMeta
from sqlalchemy.schema import Column
from sqlalchemy.types import Integer
class Meta(DeclarativeMeta):
def __init__(cls, classname, bases, dict_):
for attr in dict_.itervalues():
if isinstance(attr, Column) and attr.primary_key:
break
else:
dict_['id'] = Column(Integer, primary_key=True)
return super(Meta, cls).__init__(classname, bases, dict_)
Base = declarative_base(metaclass=Meta)
Of course, that doesn't work anymore in 0.6. The suggestion from the
aforementioned threads is to replace:
dict_['id'] = Column(Integer, primary_key=True)
with
cls.id = Column(Integer, primary_key=True)
Unfortunately, that alone doesn't work in this case. The problem is
that the Base class itself will be the first one to go through the
Meta.__init__() method, so the whole thing essentially becomes:
Base.id = Column(Integer, primary_key=True)
For it to work, I have to wrap the code in an if-block, i.e.
class Meta(DeclarativeMeta):
def __init__(cls, classname, bases, dict_):
if classname != 'Base':
for attr in dict_.itervalues():
if isinstance(attr, Column) and attr.primary_key:
break
else:
cls.id = Column(Integer, primary_key=True)
return super(Meta, cls).__init__(classname, bases, dict_)
which looks rather ugly. Is there a cleaner way to achieve this?
I didn't think metaclasses were supposed to be pretty ?Checking that you're
not the base is pretty standard metaclass stuff. If the hardcoded name
is the issue, you can look in bases:
if object not in bases:
or something more generic:
for k in cls.__mro__[1:]:
if isinstance(k, Meta):
# you're a Base subclass
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