Re: Conditionally defined model field in abstract model

Wouldn't it be simpler to have two abstract classes, one, not having the field, and the other being a sub class of the first, and just adding the field?

On Thu, Mar 21, 2013 at 5:05 PM, Alan Johnson <alan@breakrs.com> wrote:

I've got an abstract model in my project that I want to use to define a field by default on concrete subclasses, but also to allow that field to be redefined as something other than the default dynamically.  All of this works right now: 

    class classproperty(object):
        """
        Decorator for making class properties
        """
        def __init__(self, fget):
            self.fget = fget
   
        def __get__(self, owner_self, owner_cls):
            return self.fget(owner_cls)

    class BaseModel(models.Model):
        class Meta(object):
            abstract = True
   
        @classproperty
        def _special_attribute_field(self):
            return getattr(self, '_bm_special_attribute_field', 'default')
   
        @property
        def bm_special_attribute(self):
            return getattr(self, self._special_attribute_field)

...and then there are a bunch of methods that use the latter two functions to figure out which field to access. 

The problem is that right now, classes inheriting from `BaseModel` have to define the `default` field explicitly, even if they don't use `_bm_special_attribute_field` to specify something other than the default.  What I'd like to do is programmatically define `default` on concrete submodels *only if* those models don't use `_bm_special_attribute_field` to change it to something else, in which case, they should bring their own field.  Is there a way to do this, perhaps with metaclasses or something?  The key thing being that it has to not muck up the Django machinery.

(cross posted from http://stackoverflow.com/questions/15249306/django-conditionally-defined-model-field-in-abstract-model, if you want to answer for points)

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