3. Creating online poll as an app

For your information, this documentation is re-created based on https://docs.djangoproject.com/en/2.1/intro/ based on my preference.

In this section, you will learn about the concept of app, model and view.

Let us create a Python application for "online poll" based on Django. The first thing you should do is changing your working directory to your project.

$ cd my_project

STEP 1. Create an application

In order to create an app, you will need "manage.py" created by Django. Once you verify it, you can create "online poll" app by running below command.

$ python manage.py startapp polls

You will be able to see the directory structure like below if you haven't faced any error

polls/
    __init__.py
    admin.py
    apps.py
    migrations/
        __init__.py
    models.py
    tests.py
    views.py

So above directory will be your base camp for "online poll" application.

STEP2. Creating a first view

Let us open views.py and add some codes like below.

from django.shortcuts import render

# Create your views here.
from django.http import HttpResponse


def index(request):
    return HttpResponse("Hello, world. You're at the polls index.")

The above is the most simple example of view in Django. In order to call view, you should use URLconf in order to have the connected URL. What you should do is just creating a null file as urls.py. You can do it like below

$ touch urls.py

Then you should be able to see your directory structure like below

polls/
    __init__.py
    admin.py
    apps.py
    migrations/
        __init__.py
    models.py
    tests.py
    urls.py
    views.py

And let us put below content to urls.py

from django.urls import path

from . import views

urlpatterns = [
    path('', views.index, name='index'),
]

The next action you should do is adding following two lines to url.py on your project like below.

from django.conf.urls import include

path('polls/', include('polls.urls')),

So, the final code should be like below

from django.contrib import admin
from django.urls import include, path
from django.conf.urls import include


urlpatterns = [
    path('admin/', admin.site.urls),
    path('polls/', include('polls.urls')),
]

Now index view is integrated with URLconf. Time for you to check your server by following command like

$ python manage.py runserver

If you face an error like below, you should check if your python is properly installed and django is running on virtual environment like I mentioned at "1. How do I setup Django development environment on Mac?" or not.

STEP 3. Database setup

If everything goes fine, now is time for us to setup a database where can store poll results. Let us do it step by step.

1. Open up settings.py on your project, and you can see the default database settings like below
   
   

   settings.py

   # Database
   # https://docs.djangoproject.com/en/2.1/ref/settings/#databases
   
   
   DATABASES = {
       'default': {
           'ENGINE': 'django.db.backends.sqlite3',
           'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
       }
   }

   If you want to use mysql instead of sqlite3, you can use it by replacing as "django.db.backends.mysql". For oracle, it will be "django.db.backends.oracle". 
   If you are not using SQLite as your database, additional settings such as USER, PASSWORD, and HOST must be added. For more details, see the reference documentation for DATABASES.
   

   Click here to expand...

   Example)
   

   settings.py

   DATABASES = {
       'default': {
           'ENGINE': 'django.db.backends.postgresql',
           'USER': 'mydatabaseuser',
           'NAME': 'mydatabase',
           'TEST': {
               'NAME': 'mytestdatabase',
           },
       },
   }

   
   Once you set all your requirements, you should run following command.

   python manage.py migrate

2. Creating models for online poll.

   In our simple poll app, we’ll create two models: Question and Choice. A Question has a question and a publication date. A Choice has two fields: the text of the choice and a vote tally. Each Choice is associated with a Question.

   These concepts are represented by simple Python classes. Edit the polls/models.py file so it looks like this:

   polls/models.py

   from django.db import models
   
   # Create your models here.
   class Question(models.Model):
       question_text = models.CharField(max_length=200)
       pub_date = models.DateTimeField('date published')
       def __str__(self):
           return self.question_text
   
   class Choice(models.Model):
       question = models.ForeignKey(Question, on_delete=models.CASCADE)
       choice_text = models.CharField(max_length=200)
       votes = models.IntegerField(default=0)
       def __str__(self):
           return self.choice_text

   If you see above, it looks like a table schema when we create a table in database. I won't elaborate the detail about above for this time.
   
   

3. Activating models
   Even though we have created an app at the moment, but actually we can't say "polls" are correctly integrated with the project. In django, we should make some additional action to make it happen.
   To include the app in our project, we need to add a reference to its configuration class in the INSTALLED_APPS setting. The PollsConfig class is in the polls/apps.py file,

   polls/apps.py

   from django.apps import AppConfig
   
   class PollsConfig(AppConfig):
       name = 'polls'

   
   so its dotted path is 'polls.apps.PollsConfig'. Edit the <your project folder>/settings.py file and add that dotted path to the INSTALLED_APPS setting. It’ll look like this:

   <your project folder>/settings.py

   INSTALLED_APPS = [
       'polls.apps.PollsConfig',
       'django.contrib.admin',
       'django.contrib.auth',
       'django.contrib.contenttypes',
       'django.contrib.sessions',
       'django.contrib.messages',
       'django.contrib.staticfiles',
   ]

   Note that 'polls.apps.PollsConfig' is added on top of 'django.contrib.admin'.

   Now Django knows to include the polls app. Let’s run another command:

   python manage.py makemigrations polls

   As a result, you should see similar to the following

   Migrations for 'polls':
     polls/migrations/0001_initial.py
       - Create model Choice
       - Create model Question
       - Add field question to choice

   By running makemigrations, you’re telling Django that you’ve made some changes to your models (in this case, you’ve made new ones) and that you’d like the changes to be stored as a migration.

   Migrations are how Django stores changes to your models (and thus your database schema) - they’re just files on disk. You can read the migration for your new model if you like; it’s the file polls/migrations/0001_initial.py. Don’t worry, you’re not expected to read them every time Django makes one, but they’re designed to be human-editable in case you want to manually tweak how Django changes things.

   There’s a command that will run the migrations for you and manage your database schema automatically - that’s called migrate, and we’ll come to it in a moment - but first, let’s see what SQL that migration would run. The sqlmigrate command takes migration names and returns their SQL:

   python manage.py sqlmigrate polls 0001

   As a result, you should see similar to the following

   BEGIN;
   --
   -- Create model Choice
   --
   CREATE TABLE "polls_choice" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "choice_text" varchar(200) NOT NULL, "votes" integer NOT NULL);
   --
   -- Create model Question
   --
   CREATE TABLE "polls_question" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "question_text" varchar(200) NOT NULL, "pub_date" datetime NOT NULL);
   --
   -- Add field question to choice
   --
   ALTER TABLE "polls_choice" RENAME TO "polls_choice__old";
   CREATE TABLE "polls_choice" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "choice_text" varchar(200) NOT NULL, "votes" integer NOT NULL, "question_id" integer NOT NULL REFERENCES "polls_question" ("id") DEFERRABLE INITIALLY DEFERRED);
   INSERT INTO "polls_choice" ("id", "choice_text", "votes", "question_id") SELECT "id", "choice_text", "votes", NULL FROM "polls_choice__old";
   DROP TABLE "polls_choice__old";
   CREATE INDEX "polls_choice_question_id_c5b4b260" ON "polls_choice" ("question_id");
   COMMIT;

   Now, run migrate again to create those model tables in your database:

   python manage.py migrate

   You will see following similar results if everything goes fine.

   Operations to perform:
     Apply all migrations: admin, auth, contenttypes, polls, sessions
   Running migrations:
     Applying polls.0001_initial... OK

   But for now, remember the three-step guide to making model changes:

   1. Change your models (in models.py).

   2. Run python manage.py makemigrations to create migrations for those changes

   3. Run python manage.py migrate to apply those changes to the database.

4. Playing with the API
   Now, let’s hop into the interactive Python shell and play around with the free API Django gives you. To invoke the Python shell, use this command:

   python manage.py shell

   Once you’re in the shell, explore the database API:

   >>> from polls.models import Choice, Question
   >>> Question.objects.all()
   <QuerySet []>
   >>> from django.utils import timezone
   >>> q = Question(question_text="What's new?", pub_date=timezone.now())
   >>> q.save()
   >>> q.id
   1
   >>> q.question_text
   "What's new?"
   >>> q.pub_date
   datetime.datetime(2018, 11, 10, 5, 29, 23, 419109, tzinfo=<UTC>)
   >>> q.question_text = "What's up?"
   >>> q.save()
   >>> Question.objects.all()
   <QuerySet [<Question: Question object (1)>]>

   If everything is okay, you will see the same result except q.pub_date, because that will be based on your current timezone.
   In addition, let us add more features in polls/models.py like below
   
   

   polls/modes.py

   import datetime
   from django.db import models
   from django.utils import timezone
   
   # Create your models here.
   
   class Question(models.Model):
       question_text = models.CharField(max_length=200)
       pub_date = models.DateTimeField('date published')
       def __str__(self):
           return self.question_text
       def was_published_recently(self):
           return self.pub_date >= timezone.now() - datetime.timedelta(days=1)
   
   
   class Choice(models.Model):
       question = models.ForeignKey(Question, on_delete=models.CASCADE)
       choice_text = models.CharField(max_length=200)
       votes = models.IntegerField(default=0)
       def __str__(self):
           return self.choice_text

   Note that we imported datetime and timezone from django.utils to implement three additional functions on Question and Choice.
   Save these changes and start a new Python interactive shell by running python manage.py shell again:

   >>> from polls.models import Choice, Question
   >>> Question.objects.all()
   <QuerySet [<Question: What's up?>]>
   >>> Question.objects.filter(id=1)
   <QuerySet [<Question: What's up?>]>
   >>> Question.objects.filter(question_text__startswith='What')
   <QuerySet [<Question: What's up?>]>
   >>> from django.utils import timezone
   >>> current_year = timezone.now().year
   >>> Question.objects.get(pub_date__year=current_year)
   <Question: What's up?>
   >>> Question.objects.get(id=2)
   Traceback (most recent call last):
     File "<console>", line 1, in <module>
     File "/Users/..../env/lib/python3.7/site-packages/django/db/models/manager.py", line 82, in manager_method
       return getattr(self.get_queryset(), name)(*args, **kwargs)
     File "/Users/..../env/lib/python3.7/site-packages/django/db/models/query.py", line 399, in get
       self.model._meta.object_name
   polls.models.Question.DoesNotExist: Question matching query does not exist.
   >>> Question.objects.get(pk=1)
   <Question: What's up?>
   >>> q = Question.objects.get(pk=1)
   >>> q.was_published_recently()
   True
   >>> q = Question.objects.get(pk=1)
   >>> q.choice_set.all()
   <QuerySet []>
   >>> q.choice_set.create(choice_text='Not much', votes=0)
   <Choice: Not much>
   >>> q.choice_set.create(choice_text='The sky', votes=0)
   <Choice: The sky>
   >>> c = q.choice_set.create(choice_text='Just hacking again', votes=0)
   >>> c.question
   <Question: What's up?>
   >>> q.choice_set.all()
   <QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
   >>> q.choice_set.count()
   3
   >>> Choice.objects.filter(question__pub_date__year=current_year)
   <QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
   >>> c = q.choice_set.filter(choice_text__startswith='Just hacking')
   >>> c.delete()
   (1, {'polls.Choice': 1})

5. Creating an admin user
   

   First we’ll need to create a user who can login to the admin site. Run the following command:

   python manage.py createsuperuser

   Enter your desired username and press enter.

   Username: admin

   You will then be prompted for your desired email address:

   Email address: admin@example.com

   The final step is to enter your password. You will be asked to enter your password twice, the second time as a confirmation of the first.

   Password: **********
   Password (again): *********
   Superuser created successfully.

6. Start the development server
   The Django admin site is activated by default. Let’s start the development server and explore it.

   If the server is not running start it like so:
   
   

   python manage.py runserver

   Now, open a Web browser and go to “/admin/” on your local domain – e.g., http://127.0.0.1:8000/admin/. You should see the admin’s login screen:
   
   

7. Enter the admin site
   If everything is okay, you will see following screen
   
   
   
   

8. Make the poll app modifiable in the admin
   Even though we created an app, that is not appeared on the admin index page.
   Just one thing to do: we need to tell the admin that Question objects have an admin interface. To do this, open the polls/admin.py file, and edit it to look like this:

   from django.contrib import admin
   
   
   # Register your models here.
   from .models import Question
   
   admin.site.register(Question)

9. Explore the free admin functionality 
   

   Now that we’ve registered Question, Django knows that it should be displayed on the admin index page:
   
   

   Click “Questions”. Now you’re at the “change list” page for questions. This page displays all the questions in the database and lets you choose one to change it. There’s the “What’s up?” question we created earlier:
   
   Click the “What’s up?” question to edit it:
   
   Note that the form is automatically generated from the Question model. I haven't seen such a kind software model in another software languages. 

STEP4. Writing more views with Templates

In Django, web pages and other content are delivered by views. Each view is represented by a simple Python function (or method, in the case of class-based views). Django will choose a view by examining the URL that’s requested (to be precise, the part of the URL after the domain name).

This tutorial provides basic instruction in the use of URLconfs, and you can refer to URL dispatcher for more information.

1. Let’s add a few more views to polls/views.py. These views are slightly different, because they take an argument:

   polls/views.py

   def detail(request, question_id):
       return HttpResponse("You're looking at question %s." % question_id)
   
   def results(request, question_id):
       response = "You're looking at the results of question %s."
       return HttpResponse(response % question_id)
   
   def vote(request, question_id):
       return HttpResponse("You're voting on question %s." % question_id)

   Wire these new views into the polls.urls module by adding the following path() calls:

   polls/urls.py

   from django.urls import path
   
   from . import views
   
   urlpatterns = [
       # ex: /polls/
       path('', views.index, name='index'),
       # ex: /polls/5/
       path('<int:question_id>/', views.detail, name='detail'),
       # ex: /polls/5/results/
       path('<int:question_id>/results/', views.results, name='results'),
       # ex: /polls/5/vote/
       path('<int:question_id>/vote/', views.vote, name='vote'),
   ]

   Take a look in your browser, at “/polls/34/”. It’ll run the detail() method and display whatever ID you provide in the URL. Try “/polls/34/results/” and “/polls/34/vote/” too – these will display the placeholder results and voting pages.

2. Here’s one stab at a new index() view, which displays the latest 5 poll questions in the system, separated by commas, according to publication date:

   polls/urls.py

   def index(request):
       latest_question_list = Question.objects.order_by('-pub_date')[:5]
       output = ', '.join([q.question_text for q in latest_question_list])
       return HttpResponse(output)

   There’s a problem here, though: the page’s design is hard-coded in the view. If you want to change the way the page looks, you’ll have to edit this Python code. So let’s use Django’s template system to separate the design from Python by creating a template that the view can use.
   
   

3. 
   

4. 
   

5. Create a directory called templates in your polls directory. Django will look for templates in there.
   Within the templates directory you have just created, create another directory called polls, and within that create a file called index.html. In other words, your template should be at polls/templates/polls/index.html. Because of how the app_directories template loader works as described above, you can refer to this template within Django simply as polls/index.html.

   polls/templates/polls/index.html

   {% if latest_question_list %}
       <ul>
       {% for question in latest_question_list %}
           <li><a href="/polls/{{ question.id }}/">{{ question.question_text }}</a></li>
       {% endfor %}
       </ul>
   {% else %}
       <p>No polls are available.</p>
   {% endif %}

   Now let’s update our index view in polls/views.py to use the template:

   polls/views.py

   from django.http import HttpResponse
   from django.template import loader
   
   from .models import Question
   
   def index(request):
       latest_question_list = Question.objects.order_by('-pub_date')[:5]
       context = {'latest_question_list': latest_question_list}
       return HttpResponse(request, 'polls/index.html', context)

6. Raising a 404 error
   Now, let’s tackle the question detail view – the page that displays the question text for a given poll. Here’s the view:

   polls/views.py

   from .models import Question
   
   
   .
   .
   .
   
   
   def detail(request, question_id):
       try:
           question = Question.objects.get(pk=question_id)
       except Question.DoesNotExist:
           raise Http404("Question does not exist")
       return render(request, 'polls/detail.html', {'question': question})

   And you will need to have detail.html on template

   polls/templates/polls/detail.html

   <h1>{{ question.question_text }}</h1>
   <ul>
   {% for choice in question.choice_set.all %}
       <li>{{ choice.choice_text }}</li>
   {% endfor %}
   </ul>