# coding: utf-8
import copy


class ClonableMixin(object):
    def clone(self):
        """Return an identical copy of the instance with a new ID."""
        if not self.pk:
            raise ValueError('Instance must be saved before it can be cloned.')
        duplicate = copy.copy(self)
        # Setting pk to None tricks Django into thinking this is a new object.
        duplicate.pk = None
        duplicate.save()
        # ... but the trick loses all ManyToMany relations.
        for field in self._meta.many_to_many:
            source = getattr(self, field.attname)
            destination = getattr(duplicate, field.attname)
            for item in source.all():
                destination.add(item)
        return duplicate
