Having a nullable many-to-many field doesn't do anything. The declaration of a
ManyToManyField causes the creation of an intermediate table to hold the relationship. Each relation between the two tables is an entry in this new table. By definition, having no relations between two tables would mean no entries in this table. There's no use of
null=True from this property.
class User(models.Model): chat_id = models.ManyToManyField(null=True)
class User(models.Model): chat_id = models.ManyToManyField()