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

database-relations, django, django-models, one-to-many

Solution

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 `Dude`s (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 `Dude`s and `Business`es able to own multiple `PhoneNumber`s. I think this might be what you're after:

class Business(models.Model):
    ...

class Dude(models.Model):
    ...

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

Problem

I'm defining my Django models right now and I realized that there wasn't a `OneToManyField` in the model field types. I'm sure there's a way to do this, so I'm not sure what I'm missing. I essentially have something like this: ``` class Dude(models.Model): # 1 dude can have 0+ phone numbers numbers = models.OneToManyField('PhoneNumber') class PhoneNumber(models.Model): number = models.CharField() ``` In this case, each `Dude` can have multiple `PhoneNumber`s, but the relationship should be unidirectional, in that I don't need to know from the `PhoneNumber` which `Dude` owns it, per se, as I might have many different objects that own `PhoneNumber` instances, such as a `Business` for example: ``` class Business(models.Model): numbers = models.OneToManyField('PhoneNumber') ``` What would I replace `OneToManyField` (which doesn't exist) with in the model to represent this kind of relationship? I'm coming from Hibernate/JPA where declaring a one-to-many relationship was as easy as: ``` @OneToMany private List<PhoneNumber> phoneNumbers; ``` How can I express this in Django?

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