Django, have model to be related to many instances of itself

I have a model:

class Industry(models.Model):
    name = models.CharField(max_length=60, unique=True, null=False, blank=False)

I want it to have a sub_industries field that will hold all of its sub industries, which should be instances of Industry model.

I cannot be ManyToMany because each industry has only one parent industry.

Answer

You can add a ForeignKey to itself with:

class Industry(models.Model):
    name = models.CharField(max_length=60, unique=True)
    parent_industry = models.ForeignKey(
        'self',
        null=True,
        related_name='sub_industries'
    )

You can use NULL/None to express that an industry has no parent.