One-to-Many Relations

Back for more? Perfect! Part 4 of Cat Collector will take us through setting up a one-to-many (or 1:m) relationship in our database. Right now we have a bunch of cats and we can perform all basic CRUD functions on the cats. But since this is Cat Collector we really need to associate cats to their collectors, or users. Each single user will be able to have many cats in their collection, and this is what defines this as a one to many relationship: one user to many cats.

Let's Get a User!

In main_app/models.py, let's include Django's built-in User model from their auth library:

  from django.contrib.auth.models import User

We will use this user again later when we implement the ability to log in. This user will have an ID and we will use that to link the user to their cats. This means that we need to add the user_id to the Cat model so that we know which user each Cat belongs to. Because this is a key from another table, we call it a foreign key when we reference it from our Cat model. The foreign key in the Cat model connects to a user's ID which is the primary key in the User model. This connection of primary key to foreign key creates the relationship and because a cat can only ever have one collector it makes a one to many relationship.

  # main_app/models.py (in Cat model)
  ...
  user = models.ForeignKey(User, on_delete=models.CASCADE)
  ...

We should now run the python3 manage.py makemigrations command to integrate our foreign key. We will get a prompt from Django asking for one of two options. You should see something like this:

You are trying to add a non-nullable field 'user' to a cat without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Quit, and let me add a default in models.py
Select an option:

Let's choose option 1.

This will create a 'dummy' field of User that will be populated with null value row for us. We want this.

It will ask you one more time to enter a default value:

Please enter the default value now, as valid Python
The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now
Type 'exit' to exit this prompt

Go ahead and enter the number 1. This will set a row to simply 1. This will then trigger a migration file creation called XXXX_cat_user.py. Excellent!

Now run the migration by running

python3 manage.py migrate

Play with our Admin view and bask in the joy of being able to create Users and assigning Users to Cats! Make that assignment for all of your Cats.

Our cats are not being associated with users by default. Later when we implement authentication this will happen based on who is logged in. We can update our view function now, however, to make this complete.

Go to your main_app/views.py file and modify the class CatCreate view:

# main_app/views.py
class CatCreate(CreateView):
  model = Cat
  fields = '__all__'
  success_url = '/cats'

  def form_valid(self, form):
    self.object = form.save(commit=False)
    self.object.user = self.request.user
    self.object.save()
    return HttpResponseRedirect('/cats')

Also make sure to add this line up at the top:

from django.http import HttpResponseRedirect

Okay, what is this? We added a form_valid() function that will fire after Django validates the form. Inside this function we are saving the form without committing it to the database, we are adding the current user's ID onto that saved data, then we are saving it to the database, then we redirect to the cat list.

Create the User Profile URL

All the model code is in place but we can't really see any of this outside of the admin interface, which is nice for us but isn't very useful for our users. We should add a route to view one user, like a user profile page.

This will repeat the same pattern of:

1. Set up a URL in urls.py

2. Create a view in views.py

3. Make an HTML template in /templates

Let's go to our main_app/urls.py folder and update our urlpatterns:

# main_app/urls.py
urlpatterns = [
  path('', views.index, name='index'),
  path('<int:cat_id>/', views.show, name='show'),
  path('post_url/', views.post_cat, name='post_cat'),
  # add the line below...
  path('user/<username>/', views.profile, name='profile'),
]

The stuff in the angle brackets lets us grab a passed-in username and store it in a variable called username. Lets add to our main_app/views.py file:

# add this line to the imports
from django.contrib.auth.models import User

# add this new view function
def profile(request, username):
    user = User.objects.get(username=username)
    cats = Cat.objects.filter(user=user)
    return render(request, 'profile.html', {'username': username, 'cats': cats})

Lastly, we create a profile.html template to show a single User and all of the Cats they have collected:

{% extends 'base.html' %}
{% load static %}

{% block content %}

<h1>{{ username }}'s collection:</h1>

{% for cat in cats %}
<a href="/cats/{{ cat.id }}">
  <h3>{{ cat.name }}</h3>
</a>

{% endfor %}

{% endblock %}

Let's also update our cats/index.html page to allow us to inspect each user:

<!-- main_app/templates/index.html -->
{% extends 'base.html' %}
{% load static %}
{% block content %}
    <a href="/cats/create">Add a New Cat!</a>
    <hr />
    {% for cat in cats %}
      <a href='/cats/{{cat.id}}'>
        <p>Name: {{ cat.name }}</p>
      </a>
      <a href='/user/{{cat.user.username}}'>
        <p>Adopted By: {{cat.user.username}}</p>
      </a>
        {% if cat.age > 0 %}
          <p>Age: {{ cat.age }}</p>
        {% else %}
          <p>Age: Kitten</p>
        {% endif %}
        <hr />
        {% endfor %}
{% endblock %}