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Django 1:M Relationships

Django makes it easy to attach different types of data to your models that define what is saved in your database. It's easy to add integers, floats, dates, datetimes, short text, long text and lots of stuff.

Oh, here's a link to all of the fields Django provides for models, and links to their documentation on one-to-many and many-to-many relationships:

Of course, Django also has ways to hook up one-to-many and many-to-many relationships. Django establishes one-to-many relationships using a model.ForeignKey() field.

You'll need to add a ForeignKey field to a model to set up the association, then, after that's added, create and run some migrations.

Here's an example that shows how to model Arists, Albums and Songs and hook them up to each other:

from django.db import models

class Artist(models.Model):
  name = models.CharField(max_length=100)

  def __str__(self):
    return self.name

class Album(models.Model):
  title = models.CharField(max_length=100)
  artist = models.ForeignKey(Artist, on_delete=models.CASCADE)

  def __str__(self):
    return self.title

class Song(models.Model):
  title = models.CharField(max_length=100)
  album = models.ForeignKey(Album, on_delete=models.CASCADE)
  artist = models.ForeignKey(Artist, on_delete=models.CASCADE)

  def __str__(self):
    return self.title

Now that these models are defined, have Python detect the changes and use manage.py to create migrations and run them:

python3 manage.py makemigrations
python3 manage.py migrate

Now that relationships have been set up you can create models on the server and save models as fields of other models. Let's practice creating relations by firing up the Django shell. You'll need to import the models from your app to have access to them in the shell.

python3 manage.py shell
# import the application models.
from songs.models import *

# Create an artist, and make sure to save it.
# Other models can't refer to another model until it's saved and has an id.
weezer = Artist(name="Weezer")
weezer.save()

# Create an album with a reference to the artist that's been created.
pinkerton = Album(title="Pinkerton", artist=weezer)
pinkerton.save()

# Save a few songs that were on Pinkerton
song1 = Song(title="El Scorcho", album=pinkerton, artist=weezer)
song1.save()

song2 = Song(title="Getchoo", album=pinkerton, artist=weezer)
song2.save()

song3 = Song(title="No Other One", album=pinkerton, artist=weezer)
song3.save()

Run a query to grab all of the song objects and make sure they're saved and retrieved properly. Use a for loop to iterate through each of the songs and print out the songs title, the title of the album, and the name of the artist:

songs = Song.objects.all()
for song in songs:
  print(f'"{song.title}" on {song.album.title} by {song.artist.name}')

Customizing Admin Panel

User Associations

If you ever want to associate models to a specific user then you can import Django's built-in User model and reference it as a ForeignKey just like any other model.

Import Django's User model from their auth model module, then use that model when you declare a ForeignKey field.

from django.db import models
from django.contrib.auth.models import User

# Create your models here.
class Todo(models.Model):
    text = models.CharField(max_length=150)
    user = models.ForeignKey(User, on_delete=models.CASCADE, blank=True, null=True)

    def __str__(self):
        return self.text