From: <https://docs.djangoproject.com/en/2.0/ref/models/fields/#django.db.models.Field.null>

> 
> Avoid using null on string-based fields such as CharField and
>  TextField. If a string-based field has null=True, that means it has
>  two possible values for “no data”: NULL, and the empty string. In most
>  cases, it’s redundant to have two possible values for “no data;” the
>  Django convention is to use the empty string, not NULL.
> 
> 
> 

So What is better on ForeignKey Field? ForeignKey field is not string-based field, isn't it?

I wonder `what is convention for ForeignKey in django.`

null=True is better or blank=True is better? In performance, convenience or somewhat anything.