'Schedule a field update. \'objs\' must be a homogeneous iterable
collection of model instances (e.g. a QuerySet).'

def add_field_update(self, field, value, objs):

    if (not objs):
        return
    model = objs[0].__class__
    self.field_updates.setdefault(model, {}).setdefault((field, value), set()).update(objs)
