2021_05_12_WebClass-6

Web Development Class - VI

Web Development Class - VI recording: Here

May 12, 2021

Polling App in Django

Prerequisite

• Make sure that you've set up a Django Project as demonstrated in previous class. If not, you can take help from here.

What is polling app?

• It is an app (or web app) where some questions will be present.
• Each questions will have multiple options.
• For each question, visitors can vote on any one of the options.
• On the Index page of web app, there will a list of all the questions.
• For each question, there will be two options - detail and results.
• By clicking on detail link, visitor will be able to see all the options and will be able to vote.
• By clicking on result link, visitor will be able to see the results of voting for that question.

Creating a new app

• We will create a new app by the name polls.
• Run the following command in PyCharm's terminal

python manage.py startapp polls

• Now you'll notice that a new folder polls has been created. It has some python files in it -
  • views.py file - Here the views (Python functions) will be written.
  • models.py file - Here the models (tables) required for this app will be created.
• Notice that urls.py file is missing from this folder. What to do now?
• It's simple, we will create it ourselves. Create a new file urls.py in polls folder.
• Last but not the least, we will have to add this app in the INSTALLED_APPS list of settings.py file.

# In project1/settings.py file
INSTALLED_APPS = [
	'polls.apps.PollsConfig',

	# Below are the pre-installed apps which were already present
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
]

• Note - If you app's name was foo, you'd have added foo.apps.FooConfig in INSTALLED_APPS list.

Writing our first view

• So, respecting the tradition, the very first view which we'll write, will print Hello World! in our website's main page.
• Open views.py file and write the following code

# In project1/polls/views.py file
from django.shortcuts import render, HttpResponse

def hello_world(request):
	print("This is my first view! Hello World!")
	return HttpResponse("Hello World!")

• So, our very first view is ready. What do we need to do now? Any Guesses?.

Mapping view with URL

• For a view to get executed, we need to map/attach a URL to that view.
• Now, when there will be a request on that URL, our view will be called.
• Reminder - View is just a Python Function. It must be called so that it can perform the task which was expected from it.
• We want Hello World! to be displayed on the main page of our website (i.e. http://127.0.0.1:8000/).
• Go to project1/urls.py file and add the following line in the URL_PATTERNS list

# In project1/urls.py file
from django.urls import path, include
path("/", include("polls.urls")),

• The URL_PATTERNS should look like

# In project1/urls.py file
urlpatterns = [
	path("", include("polls.urls")),
	path("admin/", admin.site.urls),
]

• Now open project1/polls/urls.py and add following code

# In project1/polls/urls.py file
from django.urls import path
from polls import views

urlpatterns = [
	path("", views.hello_world),
]

Testing our first view

• Run the server by running following command in PyCharm's terminal.

python manage.py runserver

• Now visit http://127.0.0.1:8000/ on your browser and you must see Hello World! there.
• Also, see in PyCharm's terminal, you'll see This is my first view! Hello World! printed there. WHY?

Desiging database for polls app

• After this short success of running our first view, we will now move on to desiging the database for our app.
• We have already listed all the functionalities and requirements of our app at the beginning.
• Have a look at them and think what tables need to be made for our app?
• What columns should these tables have?
• Also think of Primary Keys and Foreign Keys.
• THINK YOURSELF FIRST! DO NOT SCROLL DOWN!

• We will design a simple database for this demo polling app.
• We will create a Table called Questions. It will have following attributes/columns
  • question_id Integer (Primary Key)
  • question_text String
  • publication_date Date
• We will also create a Table called Choices. It will have the following attributes/columns
  • choice_id Integer (Primary Key)
  • choice_text String
  • number_of_votes Integer
  • question (Foreign Key References Questions Table)

Recap: Django Models

• Models define the tables in the database with help of Object Oriented Programming.

• These models are basically Python classes, which represent a Table in Database.

• These classes are defined in models.py file.

• Class Variables of Model (Class) represent Attributes/Columns of Table.

• Instances/Objects of Model (Class) represent Rows/Tuple in Table.

• Object Oriented Programming	Database
  Class	Table
  Class Variables	Columns
  Instances/Objects of Class	Rows/Tuple in table

Creating models for our app

• Based on above decided design, we will now make models in project1/polls/models.py file.
• For Questions Table, write the following code

# In project1/polls/models.py
class Question(models.Model):  # Model class from models module is inherited in class Question.
    
    question_text = models.CharField(max_length=1000, verbose_name="Text")

    publication_date = models.DateField(verbose_name="Publication Date")

    def __str__(self):  #  A method is declared so that in admin panel Text of Question is displayed.
        return self.question_text

• Note that we have not added question_id field. Why? This is because for each model, Django creates a Primary Key by default. It's name is id or pk.

• For Choices Table, write the following code

# In project1/polls/models.py
class Choice(models.Model):

    choice_text = models.CharField(max_length=200, verbose_name="Choice Text")

    number_of_votes = models.IntegerField(verbose_name="Number of Votes")

    question = models.ForeignKey(Question, on_delete=models.CASCADE)

    def __str__(self):
        return self.choice_text

• models.CASCADE - Cascade means that if a table is deleted, then all it's related from in other tables is also deleted. Example - In this case when a Question is deleted from database, Choices corresponding to that question will also be deleted.

• models.DO_NOTHING - In this case on deleting a Question, Choices related to that question won't be deleted.

• models.RESTRICT - This will not allow any Question to be deleted as long as there is a Choice present for that question. If we want to delete the Question, firstly we will have to delete all the Choices related to that question.

• Now can you relate and see the similarities b/w these Python classes and Database's Tables?

• Visit this link to see all possible Field Types - Django Models Field Types

• So, are these tables created in the database? Answer is Not Yet.

• This is just the Python Code from which Django will create the tables when we order it to.

• Two remaining steps are - makemigrations and migrate.

• makemigrations - This command orders Django to write queries for our models in selected database engine in a python file. In other words, migration file is created.

  • Still these tables are NOT created in the database.
  • Django has just written the SQL queries for creating the databases in a Python file present under Migrations folder.

• migrate - This command orders Django to actually create the tables in database by running the queries that it created in makemigrations step.

• Now there's one more thing, Django comes with some preinstalled apps. Those apps also have some models of thieir own.

• When we install Django, we are already provided with migrations for these Tables. We just need to migrate them.

• So, we will follow the following steps

  • Firstly we will create tables for preinstalled Django Apps.

  python manage.py migrate
  

  • Then we will create migration files for our models defined in polls app.

  python manage.py makemigrations polls
  

  OR

  python manage.py makemigrations
  

  • Go check in Migrations folder of polls app, you'll find a file like 0001_initial.py. Don't be afraid, you can open that file and see what's inside!
  • Now finally we will again migrate. This will create the Tables Questions and Choices.

  python manage.py migrate
  

  • To see migration i.e. queries made in migration, run following command in PyCharm's terminal

  python manage.py sqlmigrate polls 0001_initial
  

• So, finally, our tables have been created. But where's the proof? How do we see the tables?

Django Admin Panel

• So now we'll introduce you to one of the most famous feature of Django, the Django Admin Panel.
• It is somewhat like PHPMyAdmin with better UI and features to manage our database.
• Generally, with other Web Frameworks, developers need to create their own Administration Panel, but Django provides us with inbuilt Admin Panel.
• Start the server

python manage.py runserver

• Visit http://127.0.0.1:8000/admin. A page will open asking for username and password.
• What to enter here? We've not created any account yet! Why is it asking for Password?
• It's because Django does not want unauthorised visitors to mess with your tables and databases as it can be catastrophic.
• So How will you enter into the Admin Panel? Turn into a superuser and Django won't stop you.
• We need to create a superuser which can access the Admin Panel without any restrictions.
• Press Ctrl + C to stop the server and run the following command in PyCharm's terminal.

python manage.py createsuperuser

• Enter following details

Username: admin
Email: admin@admin.com
Password: 
Password (again):

• Now superuser have been created successfully.
• Start the server again and visit http://127.0.0.1:8000/admin. Use the credentials of superuser to enter into Admin Panel.
• Do you see Questions and Choices tables? Have you been betrayed? The answer is NO.
• For our tables to appear on Admin Panel, we first need to register them from admin.py file.
• Write following code in project1/polls/admin.py

# In project1/polls/admin.py file
from .models import Question, Choice

admin.site.register(Question)
admin.site.register(Choice)

• Now, after the server restarts, you'll see that Questions and Choices table appear in the Admin Panel.
• Go ahead and add some questions and choices

Users Table

• Are you wondering, from where did the Users table appear in Django's Admin Panel?
• Did we create it and forgot? The answer is obviously NO.
• This table was provided to us by Django and was created when we ran the very first migrate command.
• I am sure now you're beginning to realise the power of Django - you don't even need to create Users table for your website, it's already there.
• We will see later how we can save data of users using this Users table (if time permits).

Recap: Steps involed in Making/Editing a Table/Model in Django

• Step 1 (Declaration) - We declare/edit models in models.py file.

• Step 2 (Activation) - We run python manage.py makemigrations command. Migrated files are created. These files contains queries written by Django in selected Database Engine in a .py file.

• Step 3 (Migration) - We run python manage.py migrate command. Actual Tables/Database is created.

• Step 4 - Register the models in admin.py and visit Django Admin Panel to interact with the database.

• Note - Rest of the content has been moved to next class here.

Content Contributors

• Kshitiz Srivastava

Materials

• Django Video Tutorial Playlist (English)
• Django Video Tutorial Playlist (Hindi)
• Django Models Field Types
• Python Resources