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layout: post title: "Django Model 学习笔记" date: 2021-12-30 13:43:00 +0800

categories: [Work]

Models: Define your data, essential fields, and behaviors of your data.

Each model maps to a single database table.

Quick example

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

This example model defines a Person. first_name and last_name are fields of the model.

CREATE TABLE myapp_person (
    "id" serial NOT NULL PRIMARY KEY,
    "first_name" varchar(30) NOT NULL,
    "last_name" varchar(30) NOT NULL
);

Using models

Edit your settings file and change the INSTALLED_APPS.

INSTALLED_APPS = [
    #...
    'myapp',
    #...
]

When you add new apps to INSTALLED_APPS, be sure to run manage.py migrate, optionally making migrations for them first with manage.py makemigrations.

Fields

Fields: class attributes == database fields
In-built APIs: clean, save, or delete......
name conflict

from django.db import models

class Musician(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    instrument = models.CharField(max_length=100)

class Album(models.Model):
    artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    release_date = models.DateField()
    num_stars = models.IntegerField()

Field types:

AutoField
BinaryField - max_length
BooleanField
CharField - max_length
DateField
DecimalField - max_length
EmailField - max_length = 254
IntegerField
TextField
...
And another type of fields.

Field options:

null
blank
default
help_text
unique
choices

from django.db import models

class Person(models.Model):
    SHIRT_SIZES = (
        ('S', 'Small'),
        ('M', 'Medium'),
        ('L', 'Large'),
    )
    name = models.CharField(max_length=60)
    shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)

from django.db import models

class Runner(models.Model):
    MedalType = models.TextChoices('MedalType', 'GOLD SILVER BRONZE')
    name = models.CharField(max_length=60)
    medal = models.CharField(blank=True, choices=MedalType.choices, max_length=10)

primary_key

from django.db import models

class Fruit(models.Model):
    name = models.CharField(max_length=100, primary_key=True)

>>> fruit = Fruit.objects.create(name='Apple')
>>> fruit.name = 'Pear'
>>> fruit.save()
>>> Fruit.objects.values_list('name', flat=True)
<QuerySet ['Apple', 'Pear']>

Verbose field names

first_name = models.CharField("person's first name", max_length=30)

Except for ForeignKey, ManyToManyField and OneToOneField.

verbose_name=

Many-to-one relationships: ForeignKey

from django.db import models

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

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

on_delete:

class Artist(models.Model):
    name = models.CharField(max_length=10)

class Album(models.Model):
    artist = models.ForeignKey(Artist, on_delete=models.CASCADE)

class Song(models.Model):
    artist = models.ForeignKey(Artist, on_delete=models.CASCADE)
    album = models.ForeignKey(Album, on_delete=models.RESTRICT)

>>> artist_one = Artist.objects.create(name='artist one')
>>> artist_two = Artist.objects.create(name='artist two')
>>> album_one = Album.objects.create(artist=artist_one)
>>> album_two = Album.objects.create(artist=artist_two)
>>> song_one = Song.objects.create(artist=artist_one, album=album_one)
>>> song_two = Song.objects.create(artist=artist_one, album=album_two)
>>> album_one.delete()
# Raises RestrictedError.
>>> artist_two.delete()
# Raises RestrictedError.
>>> artist_one.delete()
(4, {'Song': 2, 'Album': 1, 'Artist': 1})

Many-to-many relationships: ManyToManyField

from django.db import models

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

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

through

from django.db import models

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)
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

through_fields

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=50)

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

class Membership(models.Model):
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    inviter = models.ForeignKey(
        Person,
        on_delete=models.CASCADE,
        related_name="membership_invites",
    )
    invite_reason = models.CharField(max_length=64)

One-to-one relationships: OneToOneField

from django.db import models

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

    def __str__(self):
        return "%s the place" % self.name

class Restaurant(models.Model):
    place = models.OneToOneField(
        Place,
        on_delete=models.CASCADE,
        primary_key=True,
    )
    serves_hot_dogs = models.BooleanField(default=False)
    serves_pizza = models.BooleanField(default=False)

    def __str__(self):
        return "%s the restaurant" % self.place.name

class Waiter(models.Model):
    restaurant = models.ForeignKey(Restaurant, on_delete=models.CASCADE)
    name = models.CharField(max_length=50)

    def __str__(self):
        return "%s the waiter at %s" % (self.name, self.restaurant)

>>> p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
>>> p1.save()
>>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
>>> p2.save()
>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
>>> r.place
<Place: Demon Dogs the place>
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>
>>> from django.core.exceptions import ObjectDoesNotExist
>>> try:
>>>     p2.restaurant
>>> except ObjectDoesNotExist:
>>>     print("There is no restaurant here.")
There is no restaurant here.
>>> hasattr(p2, 'restaurant')
False

Import models from another app

from django.db import models
from geography.models import ZipCode

class Restaurant(models.Model):
    # ...
    zip_code = models.ForeignKey(
        ZipCode,
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )

Field name restrictions:

Cannot be a Python reserved word
Cannot contain more than one underscore in a row
Cannot end with an underscore

SQL reserved words, such as join, where or select, are allowed as model field names

Meta options

Model metadata is “anything that’s not a field

from django.db import models

class Ox(models.Model):
    horn_length = models.IntegerField()

    class Meta:
        ordering = ["horn_length"]
        verbose_name_plural = "oxen"

db_table: name of the database table
app_label: declare which app it belongs to

Model inheritance

abstract base model inheritance
multi-table inheritance
proxy inheritance

abstract base model inheritance

It does not generate a database table or have a manager, and cannot be instantiated or saved directly.
Fields inherited from abstract base classes can be overridden with another field or value, or be removed with None.
Base class Table is not created in This inheritance.

from django.db import models

# Create your models here.

class ContactInfo(models.Model):
    name=models.CharField(max_length=20)
    email=models.EmailField(max_length=20)
    address=models.TextField(max_length=20)

    class Meta:
        abstract=True

class Customer(ContactInfo):
    phone=models.IntegerField(max_length=15)

class Staff(ContactInfo):
    position=models.CharField(max_length=10)

multi-table inheritance

from django.db import models

# Create your models here.

class Place(models.Model):
    name=models.CharField(max_length=20)
    address=models.TextField(max_length=20)

    def __str__(self):
        return self.name


class Restaurants(Place):
    serves_pizza=models.BooleanField(default=False)
    serves_pasta=models.BooleanField(default=False)

    def __str__(self):
        return self.serves_pasta

Base class table is also created in this inheritance.
it will create a one to one field model relationship for Restaurant table from Place table.

proxy inheritance

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

class MyPerson(Person):
    class Meta:
        proxy = True

    def do_something(self):
        # ...
        pass

This style is used, if you only want to modify the Python level behaviour of the model, without changing the model’s fields.
You Inherit from base class and you can add your own properties except fields.
base class should not be abstract class.
we can not use multiple inheritance in proxy models.
The main use of this is to overwrite the main functionalities of existing model.
it always query on original model with overridden methods.