One-to-Many Relationships, thesarfo

Lets say we want a customer to have multiple addresses, we can define a one-to-many relationship by doing the following. First we need to make the field a ForeignKey instead of a OneToOneField, and then not specify a primary key. Not specifying a primary key means that we allow for duplicate primary keys to the same model/entity. See below

class Address(models.Model):
    street = models.charField(max_length=255)
    city = models.charField(max_length=255)
    customer = models.ForeignKey(Customer, on_delete=models.CASCADE)

See below of practical examples of an ecommerce one-to-many relationship

from django.db import models

# Create your models here.
class Collection(models.Model):
    title = models.CharField(max_length=255)

class Product(models.Model):
    title = models.CharField(max_length=255)
    description = models.TextField()
    price = models.DecimalField(max_digits=6, decimal_places=2)
    inventory = models.IntegerField()
    last_update = models.DateTimeField(auto_now=True)

    # 1toMany rlnshp with Collection
    collection = models.ForeignKey(Collection, on_delete=models.PROTECT)

class Customer(models.Model):
    #all part of choice fields
    MEMBERSHIP_BRONZE = 'B'
    MEMBERSHIP_SILVER = 'S'
    MEMBERSHIP_GOLD= 'G'

    MEMBERSHIP_CHOICES = [
        (MEMBERSHIP_BRONZE, 'Bronze'),
        (MEMBERSHIP_SILVER, 'Silver'),
        (MEMBERSHIP_GOLD, 'Gold'),
    ]
    first_name = models.CharField(max_length=255)
    last_name = models.CharField(max_length=255)
    email = models.EmailField(unique=True)
    phone = models.IntegerField()
    birth_date = models.DateField(null=True)
    # read about choice fields in django
    membership = models.CharField(max_length=1, choices=MEMBERSHIP_CHOICES, default=MEMBERSHIP_BRONZE)

class Order(models.Model):
    PAYMENT_STATUS_PENDING = 'P'
    PAYMENT_STATUS_COMPLETE = 'C'
    PAYMENT_STATUS_FAILED = 'F'

    PAYMENT_STATUS_CHOICES = [
        (PAYMENT_STATUS_PENDING, 'Pending'),
        (PAYMENT_STATUS_COMPLETE, 'Complete'),
        (PAYMENT_STATUS_FAILED, 'Failed'),
    ]
    placed_at = models.DateTimeField(auto_now_add=True)
    payment_status = models.CharField(max_length=1, choices=PAYMENT_STATUS_CHOICES, default=PAYMENT_STATUS_PENDING)
    # defining a 1toMany rlnshp with customer
    customer = models.ForeignKey(Customer, on_delete=models.PROTECT)

class OrderItem(models.Model):
    order = models.ForeignKey(Order, on_delete=models.PROTECT)
    product = models.ForeignKey(Product, on_delete=models.PROTECT)
    quantity = models.PositiveBigIntegerField()
    unit_price = models.DecimalField(max_digits=6, decimal_places=2)

class Address(models.Model):
    street = models.charField(max_length=255)
    city = models.charField(max_length=255)
    customer = models.OneToOneField(Customer, on_delete=models.CASCADE, primary_key=True)

class Cart(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)

class CartItem(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    cart = models.ForeignKey(Order, on_delete=models.CASCADE)
    quantity = models.PositiveSmallIntegerField()

The on_delete = models.PROTECT means that if the parent class/model is accidentally deleted, the child model will not be deleted as well.