django-docs.md

db/models

---

Many to One -> FK -> db.models.ForeignKey

• a Manufacturer makes multiple cars
• lowercase of Class manufacturer
• on_delete cascade, protect, set_null, set_default, set(), do_nothing

class Manufacturer(models.Model):
    # ...
    pass

class Car(models.Model):
    manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)

Many to Many

• Pizza has multiple Topping and Topping can be on multiple pizzas
• it doesn't matter which model has the m2mfield but should only put it on one not both
• plural lowercase of class toppings
• you can also create recursive relationships

class Topping(models.Model):
    # ...
    pass

class Pizza(models.Model):
    # ...
    toppings = models.ManyToManyField(Topping)

• intermediary
• you explicitly specify foreign keys to the models that are involved in the many-to-many relationship
• intermediate model must contain one - and only one - foreign key to the source model
• For a model which has a many-to-many relationship to itself through an intermediary model, two foreign keys to the same model are permitted, but they will be treated as the two (different) sides of the many-to-many relationship
• defining a many-to-many relationship from a model to itself, using an intermediary model, you must use symmetrical=False
• beatles.members. add() or create() or set() to create relationships with through_default={dict}
• remove() or clear()

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    # through
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):
        return self.name
# intermediary
class Membership(models.Model):
    # fk to models that are involved to m2m how to models are related
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()

Group.objects.filter(members__name

Person.objects.filter(
        group__name='The Beatles',
        membership__date_joined

# need to access a membership’s information you may do
Membership.objects.get(group=beatles, person=ringo)

# access the same information is by querying the many-to-many reverse relationship from a Person object:
ringo.membership_set.get(group=beatles)

One-to-one

• This is most useful on the primary key of an object when that object “extends” another object in some way.
• palces to resturant by inheritance because resturant is a place
• parent_link

Field name restrictions

• python reserved, dunder, end with underscore

Meta anything that's not a field

Manager. It’s the interface through which database query operations are provided to Django models and is used to retrieve the instances from the database.