13. Database and Migrations

• Reference

• Django’s ORM (Object-Relational Mapping) provides a powerful and flexible way to interact with databases.

• This tutorial will guide you through setting up models, creating and applying migrations, and managing your database schema. We’ll use a blogging application with Post, User, and Comment models as examples.

13.1. Setting Up Your Models

• Let’s start by defining our models in blog/models.py.

• Here is a table listing the supported field types in Django's django.db.models module along with examples for each

Field Type	Description	Example
CharField	A string field, for small- to large-sized strings.	name = models.CharField(max_length=100)
TextField	A large text field. Use this for large amounts of text.	description = models.TextField()
IntegerField	An integer field.	age = models.IntegerField()
FloatField	A floating-point number field.	price = models.FloatField()
BooleanField	A boolean field.	is_active = models.BooleanField(default=True)
DateField	A date field.	birth_date = models.DateField()
DateTimeField	A date and time field.	created_at = models.DateTimeField(auto_now_add=True)
TimeField	A time field.	start_time = models.TimeField()
EmailField	An Email field.	email = models.EmailField()
URLField	A URL field.	website = models.URLField()
SlugField	A slug field.	slug = models.SlugField()
FileField	A file upload field.	file = models.FileField(upload_to='uploads/')
ImageField	An image upload field.	image = models.ImageField(upload_to='images/')
ForeignKey	A many-to-one relationship.	author = models.ForeignKey(Author, on_delete=models.CASCADE)
OneToOneField	A one-to-one relationship.	profile = models.OneToOneField(Profile, on_delete=models.CASCADE)
ManyToManyField	A many-to-many relationship.	categories = models.ManyToManyField(Category)
DecimalField	A fixed-precision decimal number field.	price = models.DecimalField(max_digits=10, decimal_places=2)
PositiveIntegerField	An integer field for positive values only.	quantity = models.PositiveIntegerField()
PositiveSmallIntegerField	A small integer field for positive values only.	rank = models.PositiveSmallIntegerField()
SmallIntegerField	A small integer field.	votes = models.SmallIntegerField()
DurationField	A field for storing periods of time.	duration = models.DurationField()
UUIDField	A field for storing universally unique identifiers.	uuid = models.UUIDField(default=uuid.uuid4, editable=False)
BinaryField	A field for storing binary data.	data = models.BinaryField()
JSONField	A field for storing JSON data.	metadata = models.JSONField()

13.1.1. Post Model

• CharField: Represents a short-to-mid-sized string.

• ForeignKey: Creates a many-to-one relationship to the User model.

• TextField: Represents large text fields.

• DateTimeField: Represents a date and time field, with default set to the current time.

  from django.db import models
  from django.utils import timezone
  from django.contrib.auth.models import User
  
  class Post(models.Model):
      title = models.CharField(max_length=80)
      author = models.ForeignKey(User, on_delete=models.CASCADE)
      content = models.TextField()
      date_posted = models.DateTimeField(default=timezone.now)
  
      def __str__(self):
          return self.title
  
  

13.1.2. Comment Model

• ForeignKey to Post and User: Each comment is associated with a specific post and user.

  class Comment(models.Model):
      post = models.ForeignKey(Post, on_delete=models.CASCADE)
      author = models.ForeignKey(User, on_delete=models.CASCADE)
      content = models.TextField()
      date_posted = models.DateTimeField(default=timezone.now)
  
      def __str__(self):
          return f'Comment by {self.author} on {self.post}'
  

13.2. Creating and Applying Migrations

• After defining your models, you need to create and apply migrations to update the database schema.

13.2.1. Creating Migrations

• Run the following command to create migrations based on the changes in your models

• This command generates a migration file in the blog/migrations directory.

  python manage.py makemigrations
  

13.2.2. Applying Migrations

• To apply the migrations and update the database schema, run:

• This command applies the migrations to your database, creating the necessary tables and fields.

  python manage.py migrate
  

13.3. Creating Demo Data

• First, ensure you have access to Django’s shell to execute Python code within the context of your Django project. Run the following command:

  python manage.py shell
  

• Within the shell, you can create instances of your models.

  from django.contrib.auth.models import User
  from blog.models import Post, Comment
  from django.utils import timezone
  
  # Create a user
  user = User.objects.create_user(username='john_doe', email='john@example.com', password='password123')
  
  # Create some posts
  post1 = Post.objects.create(title='First Post', author=user, content='This is the content of the first post', date_posted=timezone.now())
  post2 = Post.objects.create(title='Second Post', author=user, content='This is the content of the second post', date_posted=timezone.now())
  
  # Create comments for post1
  comment1 = Comment.objects.create(post=post1, author=user, content='This is the first comment on the first post', date_posted=timezone.now())
  comment2 = Comment.objects.create(post=post1, author=user, content='This is the second comment on the first post', date_posted=timezone.now())
  
  # Create a comment for post2
  comment3 = Comment.objects.create(post=post2, author=user, content='This is the first comment on the second post', date_posted=timezone.now())
  

• If you facing any problem, you can re-create database, but your all data will be gone (including super-user)

  • In default, Django use db.sqlite3 to store all data

    rm db.sqlite3
    

13.4. ORM Queries

13.4.1. Basic Queries

1. Retrieve all posts

2. Filter posts by author

3. Get a single post by ID

   # Retrieve all posts
   all_posts = Post.objects.all()
   # Filter posts by author
   author_posts = Post.objects.filter(author=user)
   # Get a single post by ID
   single_post = Post.objects.get(id=1)
   

13.4.2. Related Object Queries

1. Get all comments for a specific post

2. Access related objects using reverse relationships

   # Get all comments for a specific post
   post_comments = Comment.objects.filter(post=post1)
   # Access related objects using reverse relationships
   post_comments = post1.comment_set.all()
   

13.4.3. Advanced Queries [field lookups]

• Django’s ORM uses field lookups, which are used to create complex queries.

• Here’s a table of common field lookups with their purpose and examples:

Function	Purpose	Example
exact	Exact match	Post.objects.filter(title__exact='First Post')
iexact	Case-insensitive exact match	Post.objects.filter(title__iexact='first post')
contains	Contains substring	Post.objects.filter(content__contains='Django')
icontains	Case-insensitive contains	Post.objects.filter(content__icontains='django')
in	Value is in a given list	Post.objects.filter(id__in=[1, 2, 3])
gt	Greater than	Post.objects.filter(date_posted__gt=timezone.now() - timedelta(days=7))
gte	Greater than or equal to	Post.objects.filter(date_posted__gte=timezone.now() - timedelta(days=7))
lt	Less than	Post.objects.filter(date_posted__lt=timezone.now())
lte	Less than or equal to	Post.objects.filter(date_posted__lte=timezone.now())
startswith	Starts with substring	Post.objects.filter(title__startswith='Fir')
istartswith	Case-insensitive starts with substring	Post.objects.filter(title__istartswith='fir')
endswith	Ends with substring	Post.objects.filter(title__endswith='Post')
iendswith	Case-insensitive ends with substring	Post.objects.filter(title__iendswith='post')
range	Value is within a range	Post.objects.filter(date_posted__range=[start_date, end_date])
date	Match a specific date	Post.objects.filter(date_posted__date=datetime.date(2023, 6, 17))
year	Match a specific year	Post.objects.filter(date_posted__year=2023)
month	Match a specific month	Post.objects.filter(date_posted__month=6)
day	Match a specific day	Post.objects.filter(date_posted__day=17)
week_day	Match a specific day of the week	Post.objects.filter(date_posted__week_day=2)
isnull	Check for NULL (or not NULL)	Post.objects.filter(author__isnull=True)

1. Retrieve posts containing a keyword

2. Retrieve posts published in the last 7 days

3. Order posts by date posted (newest first)

   # Retrieve posts containing a keyword
   keyword_posts = Post.objects.filter(content__icontains='content')
   # Retrieve posts published in the last 7 days
   recent_posts = Post.objects.filter(date_posted__gte=timezone.now() - timezone.timedelta(days=7))
   # Order posts by date posted (newest first)
   ordered_posts = Post.objects.all().order_by('-date_posted')
   

13.5. Working with the Admin Interface

• Register your models with the admin interface to manage them through Django’s admin panel.

13.5.1. Registering Models

• In blog/admin.py, register your models:

• This will make the Post and Comment models accessible in the admin interface.

  from django.contrib import admin
  from .models import Post, Comment
  
  admin.site.register(Post)
  admin.site.register(Comment)