Updating mixin subclassed instances

[Radek Svarz]

Hi,

I am using my custom permission mixin on several of my apps's models (not all).

For the case when I need to merge from the "old - context" permission to the "new - context" permission I want to have function which changes the corresponding permission reference attribute in all models, which subclassed this mixin.

How do I know which models subclassed this permission mixin and that they have the inherited permission reference attribute?

In fact I want to have such function in my mixin:

@classmethod
def merge_to(cls, from_perm_context, to_perm_context):

    perm_context_models = []  # How to get this?

    try:
        with transaction.atomic():
            for model in perm_context_models:
                model.objects.filter(
                    perm_context=from_perm_context,
                ).update(perm_context=to_perm_context)
    except IntegrityError as e:  # or DatabaseError
        raise e

Thanks, 

Radek

[Vijay Khemlani]

You could use a metaclass to keep track of all the classes that inherit from a given one, in a pure Python way it would be something like this

class PluginMeta(type): # we use __init__ rather than __new__ here because we want # to modify attributes of the class *after* they have been # created def __init__(cls, name, bases, dct): if not hasattr(cls, 'registry'): # this is the base class. Create an empty registry cls.registry = {} else: # this is a derived class. Add cls to the registry interface_id = name.lower() cls.registry[interface_id] = cls super(PluginMeta, cls).__init__(name, bases, dct) class Plugin(object): __metaclass__ = PluginMeta

So, all the classes that inherit from Plugin are stored in Plugin.registry. Not sure if it works for mixins though.

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[Radek Svarz]

@Vijay, thanks. I also had a similar idea, but I was surprised it was not somewhere already implemented. 

Then I found __subclasses__() as mentioned here http://stackoverflow.com/questions/40353219/updating-django-mixin-subclassed-instances/40353284#40353284

It works (Python 3.3, Django 1.8) ;)

Thx, R

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