Relationships

I started getting bored with looking at student pull requests that hardly ever included more than one model. So I decided to make their lives a little more challenging. I'll admit, it was mostly for my benefit, but hey, it's good for them to experiment with relating models to each other too! So now, they are asked to add at least 2 related models to their apps. A little optional challenge.

In Django, there are 3 different fields you can use to represent relationships between models:

1. ForeignKey

This represents a many-to-one relationship. Like pets to a household. This field requires 2 arguments- the related class(the model it is related to) and the on_delete option. So it looks like this:

class Pet(models.Model):

    owner = models.ForeignKey('Owner', on_delete=models.CASCADE)

class Owner(models.Model):

    [insert whatever fields you choose to include in this class here]

On delete >>> cascade is an example often used online. But there are more options than just cascade. Django documentation says you can  CASCADE/ PROTECT/ RESTRICT/ SET_NULL/ SET_DEFAULT/ SET()/ DO_NOTHING. This site provides a pretty good explanation of what these options mean. You can also look at the official Django Docs . 

2. ManyToManyField

This represents a many-to-many relationship. Like pizza and toppings. It has one required argument- the related class(model). When you run the command 'python manage.py migrate', you'll notice that an intermediary join table is generated by Django to represent this relationship. It is named using the field name and the name of the model.

class Topping(models.Model):

    topping = models.ManyToManyField('Pizza')

class Pizza(models.Model):

    [insert whatever fields you choose to include in this class here]

This field also accepts optional arguments and you can read about them in the official Django Docs. 

3. OneToOneField

This represents a one-to-one relationship. No surprises there. According to the Django docs, you can sort of create this relationship with a ForeignKey as long as you indicate that unique=True. The difference is that the relationship will always return a single object. Whereas when using a ForeignKey, a list will be returned. One argument is required- the related class. You can specify a related_name argument. But if you don't, Django will generate one for you using the lowercase name of the current model.

class Toothbrush(models.Model):

    toothbrush = models.OneToOneField('User')

class User(models.Model):

    [insert whatever fields you choose to include in this class here]

Part of designing your database schema is determining how your models should relate to each other. I'm still trying to learn and better understand this better.