Django_Quick_Revision_Notes_Part1

# Django Notes:
## Some Django Commands Definition:
First of all,

python manage.py makemigrations appname

will create (migration_number).py files in your app migrations folders. These lines of code are nothing but statements which help in creating actual fields in your respective database similar to SQL statements.

In order to execute the migration which was created using the previous command, we will run the following command,

python manage.py migrate

On migrate your new model fields will be reflected in database if there are no errors

## Note:

1. When you add/change model methods, then you don't need to run ./manage makemigrations and ./manage.py migrate.

But whenever you edit your model fields (adding a new one, changing an existing one or altering any of the arguments it takes) then you should always run migrations.

2. We use {% %} and {{ }} to put python code and python variable html code in django respectively and this is known as jinja formatting.
For loop and if statements end with {% endfor %} and {% endif %} in jinja formatting.

3. Steps to add model fields of a class in django admin panel:

  a). Go to the app contaning that model.

  b). Go to its admin.py file 

  c). Import the model

  d). write

		from django.contrib import admin
		admin.site.register(model_name)

4. If you want that a class return a string on calling instead of class type then use dunder (double underscore) str built in function of python.

for eg.
		
		class album(models.Model):
			def __str__(self):
				return self.artist

5. Regex to extract serial number (integer value) from url:

		r'^(?P<album_id>[0-9]+)/$'

    Here, ^ (carrot sign) and $ represents starting and ending of regex. 

6.  Shortcut way to rise 404 Error:

	from django.shortcuts import get_object_or_404
	album = get_object_or_404(Album , pk = album_id)
        
It will throw a message of Not found error on screen if album doesn't exists inside database.  


## Django Imports:

	  from django.db import models
	  from django.http import Http404
	  from django.http import HttpResponse
	  from django.shortcuts import render
	  from django.contrib import admin

## Django Database Query Shell Commands:
- from app.models import album, song (Name of model)
- Command to see all objects inside album

		album.objects.all()

- Way to create a new object inside album:

    1.

		a = album(artist="Weeknd",album_title="Star_boy", songn	me="star_boy") 
		a.save()

    2.

		album.objects.create(artist ="Weeknd", album_title 
		="Star_boy", song = "star_boy")

- To filter an object using an attribute.

		album.objects.filter(id = 1)
	
               Here id or pk is the default primary use by django, It's like 
               serial no
      
        Another eg:

		album.objects.filter(artist__startswith='Week')

- Way to get an object from database on a specific attribute:

		album.objects.get(artist="Weekend")

- Some other useful commands:

		album1 = album.objects.get(pk=1)

		album1.song_set.all()   

   This will print all song class objects
   under album1 object.
   Note that class name should be in lowercase (song) always.

		album1.song_set.create(song_title = 'I love 	bacon',file_type='mp3')    

 This will create a song class object
 under album1 object.

		album1.song_set.count()

 This will count the number of song class objects under
 album1 object.


## Django Views

- Rendering

    Way to render a template on a request:

     1.Define a function in views and on url request, call an view 
     function.

    2.Ex. of a view function
      
		def index(request):
			context = {'all_albums': all_albums}
			return render(request,
			'path_to_template('music/index.html'), context)
            
                  Here, context is a dictionary which replaces the words  in 
                  jinja code with outside values  or data
             
                 
## Using Dynamic Url
  Pass the url by name instead of url pattern. This will do the future 
  proofing. For eg, If you want to change the url patterns then you 
  have to modified it only at one place instead of passing it to every 
  point.
  Eg. of  Dynamic url passing:

		{% url 'detail' album.id %}
		
		where detail is the url name and album.id is replacing the v
		ariable with value

 Where urlpatterns is:

		url(r'^(?P<album_id>[0-9]+)/$', views.detail, 
                name='detail')

But, if more than one url pattern of different app has same name then we have to use namespace standard
we pass an statement in urls.py file of the app as follows:

		app_name = 'music'

Now, modified dynamic url passing is as follows:

		{% url 'music:detail' album.id %}

This will tell django that it has to see url name 'detail'  inside music app.