In Model-Template-View paradigm, the model determines the structure and type of data, the view is the data to be presented, and the template is how the data is presented.
The model is the single source of information containing essential fields and behavior of data.
from django.db import models class Person(models.Model): first_name = models.CharField(max_length=30) last_name = models.CharField(max_length=30)
Django provides database storage that can be used with the Django programming interface.
Django will by default create an SQLite database in the file db.sqlite3.
└── myproject/ └── myproject/ └── manage.py └── db.sqlite3
Mapping Models to Databases
Each model attribute maps to a column in a relational database table. In the provided example, see the raw SQL created that Django translated from a
CREATE TABLE myapp_car ( id integer NOT NULL PRIMARY KEY AUTOINCREMENT, make varchar(20) NOT NULL, model varchar(20) NOT NULL, year date NOT NULL );
Models are defined in models.py within the app’s folder.
from django.db import models class Car(models.Model): make = models.CharField(max_length=20) model = models.CharField(max_length=20) year = models.DateField()
A primary key is a column that uniquley identifies each row in a relational database.
class Car(models.Model): make = models.CharField(max_length=100, primary_key=True)
ForeignKey field type setups up a one-to-many relationship between models.
class Author(models.Model): name = models.CharField(max_length=64) # Book has a many-to-one relationship with Author class Book(models.Model): isbn = models.CharField(max_length=15, primary_key=True) title = models.CharField(max_length=255) author = models.ForeignKey(Author, on_delete=models.CASCADE)
Models come with predefined methods that are inherited from the
Model parent class. These models are invoked on the model instances, not the model class.
Every model can be defined with optional metadata, which is anything that is not a field.
class Person(models.Model): age = models.IntegerField() class Meta: ordering = ["age"] verbose_name_plural = "people"
Every model class has a metadata option,
verbose_name, which serves as a human-readable class name.
first_name = models.CharField("person's first name", max_length=30)
Default values can be added as optional arguments in models.
from django.db import models class Applicant(models.Model): GRADE = [ ('P', 'Pass'), ('F', 'Fail') ] grade = models.CharField(max_length=1, choices=GRADE, default='P') "
Choices in Models
choices key can be used by providing an array of tuples where the first value is the actual value to be set in the database, and the second value is a human-readable name.
from django.db import models class Applicant(models.Model): SINGLE = 'S' MARRIED = 'M' DIVORCED = 'D' STATUS = [ (SINGLE, 'Single'), (MARRIED, 'Married'), (DIVORCED, 'Divorced') ] marital_status = models.CharField(max_length=1, choices=STATUS, default=SINGLE)
There are two consecutive steps to perform migrations. First, the
makemigrations command will commit model changes into migration files. Then the
migrate command can be used to apply changes to the database schema.
python3 manage.py makemigrations python3 manage.py migrate
makemigrations command packages any model schema changes inside models.py into migration files. The name of an app is optional but is helpful when there are multiple apps in a project.
python3 manage.py makemigrations
Migration files are created in the migrations directory.
└── myproject/ └── migrations/
Migrations can be applied to an existing database schema and an app name can be provided to target a specific database schema.
python3 manage.py migrate myapp
Rolling Back Migrations
Migrations can be rolled back to a specific migration using the
migrate command and providing the name of the migration file to roll back to as an argument.
python3 manage.py migrate 0001
Basic database operations are commonly referred to as CRUD- Create, Read, Update, and Delete.
Structured Query Language
The underlying language for CRUD operations is Structured Query Language, SQL.
CRUD operations can be executed in the Python shell.
Models must be imported into the Python shell to be used.
from app_name.models import ModelName
An instance of a model can be created by calling the model with the fields as arguments.
model_instance = ModelName(field1="field 1 value")
A model instance can be saved to the database using the
Viewing All Instances
All the instances of a model can be returned as a QuerySet using the
model_instance = ModelName.objects.all()
A QuerySet is a collection of objects from a database.
A QuerySet object can be retrieved using bracket notation.
c = Client.objects.all() obj = c
The first instance of a model can be retrieved using the
.first() query method.
first_instance = ModelName.objects.first()
A model instance’s field value can be accessed using dot notation.
Updating Field Value
A field’s value can be reassigned using dot notation.
mode_instance.field_name = "New value"
Save Model Instance
An updated model instance must be saved into the database using the
The reverse relationship can be queried using the
._set property, where the
_set property is preceded by the lowercase name of the model.
Deleting Model Instance
Model instances can be deleted using the
.CASCADE must be implemented on the model itself and will delete everything related to the instance of that model.
model_instance = models.ForeignKey(ModelType, null=True, on_delete=models.CASCADE)
.get() method returns a single object that matches the arguments provided. If
.get() matches multiple objects, a
.MultipleObjectsReturned exception will return.
skate_blog = Blog.objects.get(name="Skating Post")
.get_or_create() method will return a tuple that contains an object, with the supplied arguments, and a boolean that states whether the object was created or not.
(<ModelName: ModelName object (15)>, True)
.exclude() method returns all objects that do not match provided the arguments.
.order_by() method returns a list of objects based on a specified order which is provided as an argument.
Foreign Keys can connect two tables together through a one-to-many relationship.
class Book (models.Model): author = models.ForeignKey('Author',on_delete=models.CASCADE)
.filter() method is used to search for a model instance related to another model instance.