Relationships and Forms in Django

Relationships in Django

Django has a number of built-in relationships that you can use to connect your models to each other. The most common relationships are:

One-to-many
Many-to-many
One-to-one

One-to-many

One-to-many relationships are used when you have a model that has a foreign key to another model. In a One-to-Many relationship, each instance of the parent model can be associated with multiple instances of the child model. For example, a Chai variety can have multiple reviews.

In the existing chai app, open models.py and add the following code:

class ChaiReview(models.Model):
    chai = models.ForeignKey(ChaiVariety, on_delete=models.CASCADE, related_name='reviews')
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    rating = models.IntegerField()
    comment = models.TextField()
    date_added = models.DateTimeField(default=timezone.now)

    def __str__(self):
        return f'{self.user.username} review for {self.chai.name}'

This code creates a new model called ChaiReview that has a foreign key to the ChaiVariety model. The user field is a foreign key to the User model, and the rating and comment fields are integers and text fields respectively.

Many-to-many

Many-to-many relationships are used when you have a model that has a many-to-many relationship with another model. In a Many-to-Many relationship, each instance of one model can be associated with multiple instances of another model, and vice versa. For example, a Chai variety can be featured in multiple stores, and each store can feature multiple chai varieties.

In the existing chai app, open models.py and add the following code:

class Store(models.Model):
    name = models.CharField(max_length=100)
    location = models.CharField(max_length=100)
    chai_varieties = models.ManyToManyField(ChaiVariety, related_name='stores')

    def __str__(self):
        return self.name

One-to-one

One-to-one relationships are used when you have a model that has a one-to-one relationship with another model. In a One-to-One relationship, each instance of one model is associated with one and only one instance of another model. For example, each Chai variety can have a unique certificate.

In the existing chai app, open models.py and add the following code:

class ChaiCertificate(models.Model):
    chai = models.OneToOneField(ChaiVariety, on_delete=models.CASCADE, related_name='certificate')
    certificate_number = models.CharField(max_length=100)
    issued_date = models.DateTimeField(default=timezone.now)
    valid_until = models.DateTimeField()

    def __str__(self):
        return f'Certificate for {self.chai.name}'

Update the admin

In the existing chai app, open admin.py and add the following code:

from django.contrib import admin
from .models import ChaiVariety, ChaiReview, Store, ChaiCertificate

class ChaiReviewInline(admin.TabularInline):
    model = ChaiReview
    extra = 1

class ChaiVarietyAdmin(admin.ModelAdmin):
    list_display = ('name', 'type', 'date_added')
    inlines = [ChaiReviewInline]

class StoreAdmin(admin.ModelAdmin):
    list_display = ('name', 'location')
    filter_horizontal = ('chai_varieties',)

class ChaiCertificateAdmin(admin.ModelAdmin):
    list_display = ('chai', 'certificate_number', 'issued_date', 'valid_until')

admin.site.register(ChaiVariety, ChaiVarietyAdmin)
admin.site.register(Store, StoreAdmin)
admin.site.register(ChaiCertificate, ChaiCertificateAdmin)

Adding a form on frontend

In the existing chai app, create a new file called forms.py in the chai app directory. In this file, add the following code:

from django import forms
from .models import ChaiVariety


class ChaiVarietyForm(forms.Form):
  chai_variety = forms.ModelChoiceField(queryset=ChaiVariety.objects.all(), label="Select Chai Variety")

Handle the view for the form

In the existing chai app, open views.py and add the following code:

from .models import ChaiVariety, Store
from .forms import ChaiVarietyForm

def chai_store_view(request):
  stores = None
  if request.method == 'POST':
    form = ChaiVarietyForm(request.POST)
    if form.is_valid():
      chai_variety = form.cleaned_data['chai_variety']
      stores = Store.objects.filter(chai_varieties=chai_variety)
  else:
    form = ChaiVarietyForm()

  return render(request, 'chai/chai_stores.html', {'form': form, 'stores': stores})

Add the template

In the existing chai app, create a new file called chai_stores.html in the chai app directory. In this file, add the following code:

{% extends 'layout.html' %}

{% block content %}
  <h1>Chai Stores</h1>
  <form method="post">
        {% csrf_token %}
        {{ form.as_p }}
        <button type="submit">Search Stores</button>
    </form>
  {% if stores %}
        <h2>Stores with selected Chai Variety</h2>
        <ul>
            {% for store in stores %}
                <li>{{ store.name }} - {{ store.location }}</li>
            {% endfor %}
        </ul>
    {% endif %}
{% endblock %}

Update the urls

In the urls.py file, add the following code to the urlpatterns list:

path('chai_stores/', views.chai_store_view, name='chai_stores'),

Run the server

In the terminal, navigate to the chai directory and run the following command:

python manage.py runserver

That’s it! You have successfully created a form that allows users to search for stores that have a specific chai variety. You can now add more functionality to the form and the view to make it more useful.

Conclusion

In this section, you learned how to create a form that allows users to search for stores that have a specific chai variety. We have also discussed about the different types of relationships in Django and how to use them in your models and views. Hope this was helpful! Stay tuned for the next part of the tutorial where we will learn how to create a form that allows users to add a new chai variety to the database.