[Answer]-Django model column

From the documentation for default:

This can be a value or a callable object. If callable it will be called every time a new object is created.

So, you can pass it a function:

_random_id = models.CharField(max_length=18, default=f)

Q is the name of a built-in in django, so its best not to use it for your model.

Further,

@property
def random_id(self):
    return self._random_id

This is only serving as syntactical sugar because you can refer to the _random_id field directly from an instance of the model:

foo = Q()
foo._random_id

So you should consider removing this property entirely; and if you need random_id, set it as the name of your field.