How to express a One-To-Many relationship in Django?

Question

To handle One-To-Many relationships in Django you need to use ForeignKey.

The documentation on ForeignKey is very comprehensive and should answer all the questions you have:

https://docs.djangoproject.com/en/3.2/ref/models/fields/#foreignkey

The current structure in your example allows each Dude to have one number, and each number to belong to multiple Dudes (same with Business).

If you want the reverse relationship, you would need to add two ForeignKey fields to your PhoneNumber model, one to Dude and one to Business. This would allow each number to belong to either one Dude or one Business, and have Dudes and Businesses able to own multiple Numbers. I think this might be what you are after.

class Business(models.Model):
    ...
class Dude(models.Model):
    ...
class PhoneNumber(models.Model):
    dude = models.ForeignKey(Dude)
    business = models.ForeignKey(Business)

In Django, a one-to-many relationship is called ForeignKey. It only works in one direction, however, so rather than having a number attribute of class Dude you will need

class Dude(models.Model):
    ...

class PhoneNumber(models.Model):
    dude = models.ForeignKey(Dude)

Many models can have a ForeignKey to one other model, so it would be valid to have a second attribute of PhoneNumber such that

class Business(models.Model):
    ...
class Dude(models.Model):
    ...
class PhoneNumber(models.Model):
    dude = models.ForeignKey(Dude)
    business = models.ForeignKey(Business)

You can access the PhoneNumbers for a Dude object d with d.phonenumber_set.objects.all(), and then do similarly for a Business object.

To be more clear - there's no OneToMany in Django, only ManyToOne - which is Foreignkey described above. You can describe OneToMany relation using Foreignkey but that is very inexpressively.

A good article about it: https://amir.rachum.com/blog/2013/06/15/a-case-for-a-onetomany-relationship-in-django/

You can use either foreign key on many side of OneToMany relation (i.e. ManyToOne relation) or use ManyToMany (on any side) with unique constraint.