models.py

def _inherited(parent_attr, direct_attr):
    """Return a descriptor delegating to an attr of the original document.

    If `self` is a translation, the descriptor delegates to the attribute
    `parent_attr` from the original document. Otherwise, it delegates to the
    attribute `direct_attr` from `self`.

    Use this only on a reference to another object, like a ManyToMany or a
    ForeignKey. Using it on a normal field won't work well, as it'll preclude
    the use of that field in QuerySet field lookups. Also, ModelForms that are
    passed instance=this_obj won't see the inherited value.

    """
    getter = lambda self: (getattr(self.parent, parent_attr)
                               if self.parent and
                                  self.parent.id != self.id
                           else getattr(self, direct_attr))
    setter = lambda self, val: (setattr(self.parent, parent_attr,
                                        val) if self.parent and
                                                self.parent.id != self.id else
                                setattr(self, direct_attr, val))
    return property(getter, setter)