[Django] (fields.W340) null has no effect on ManyToManyField.

Warning

model name:  (fields.W340) null has no effect on ManyToManyField.

null has no effect since there is no way to require a relationship at the database level.

ManyToManyの実態は、リレーションテーブルなので、そのレコードの中でnullを許容するということが物理的にできないです。

https://stackoverflow.com/questions/49677910/filter-data-from-two-tables-having-many-to-many-relation-django

解決策

field_name = models.ManyToManyField(ModelName, blank=True, null=True, related_name='n')
↓
field_name = models.ManyToManyField(ModelName, blank=True, related_name='n')