ListView – Class Based Views Django

List View refers to a view (logic) to display multiple instances of a table in the database. We have already discussed the basics of List View in List View – Function based Views Django. Class-based views provide an alternative way to implement views as Python objects instead of functions. They do not replace function-based views, but have certain differences and advantages when compared to function-based views: 

• Organization of code related to specific HTTP methods (GET, POST, etc.) can be addressed by separate methods instead of conditional branching.
• Object oriented techniques such as mixins (multiple inheritance) can be used to factor code into reusable components.

Class based views are simpler and efficient to manage than function-based views. A function based view with tons of lines of code can be converted into a class based view with few lines only. This is where Object Oriented Programming comes into impact. 

Django List View – Function Based Views

Illustration of How to create and use List view using an Example. Consider a project named geeksforgeeks having an app named geeks. 

Refer to the following articles to check how to create a project and an app in Django. 

• How to Create a Basic Project using MVT in Django?
• How to Create an App in Django ?

After you have a project and an app, let’s create a model of which we will be creating instances through our view. In geeks/models.py, 

Python3

# import the standard Django Model
# from built-in library
from django.db import models

# declare a new model with a name "GeeksModel"
class GeeksModel(models.Model):

    # fields of the model
    title = models.CharField(max_length = 200)
    description = models.TextField()

    # renames the instances of the model
    # with their title name
    def __str__(self):
        return self.title

After creating this model, we need to run two commands in order to create Database for the same. 

Python manage.py makemigrations
Python manage.py migrate

Now let’s create some instances of this model using shell, run form bash, 

Python manage.py shell

Enter following commands  

>>> from geeks.models import GeeksModel
>>> GeeksModel.objects.create(
                       title="title1",
                       description="description1").save()
>>> GeeksModel.objects.create(
                       title="title2",
                       description="description2").save()
>>> GeeksModel.objects.create(
                       title="title2",
                       description="description2").save()

Now we have everything ready for back end. Verify that instances have been created from http://localhost:8000/admin/geeks/geeksmodel/ 

Class Based Views automatically setup everything from A to Z. One just needs to specify which model to create ListView for, then Class based ListView will automatically try to find a template in app_name/modelname_list.html. In our case it is geeks/templates/geeks/geeksmodel_list.html. Let’s create our class based view. In geeks/views.py, 

Python3

from django.views.generic.list import ListView
from .models import GeeksModel

class GeeksList(ListView):

    # specify the model for list view
    model = GeeksModel

Now create a url path to map the view. In geeks/urls.py, 

Python3

from django.urls import path

# importing views from views..py
from .views import GeeksList
urlpatterns = [
    path('', GeeksList.as_view()),
]

Create a template in templates/geeks/geeksmodel_list.html, 

HTML

<ul>
    <!-- Iterate over object_list -->
    {% for object in object_list %}
    <!-- Display Objects -->
    <li>{{ object.title }}</li>
    <li>{{ object.description }}</li>

    <hr/>
    <!-- If object_list is empty  -->
    {% empty %}
    <li>No objects yet.</li>
    {% endfor %}
</ul>

Let’s check what is there on http://localhost:8000/

Manipulate Queryset in ListView

By default ListView will display all instances of a table in the order they were created. If one wants to modify the sequence of these instances or the ordering, get_queryset method need to be overridden. 
In geeks/views.py, 

Python3

from django.views.generic.list import ListView
from .models import GeeksModel

class GeeksList(ListView):

    # specify the model for list view
    model = GeeksModel

    def get_queryset(self, *args, **kwargs):
        qs = super(GeeksList, self).get_queryset(*args, **kwargs)
        qs = qs.order_by("-id")
        return qs

Now check, if the order of instances has been reversed. 

This way one can modify the entire queryset in any manner possible.