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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