Django Models

A Django model is the built-in feature that Django uses to create tables, their fields, and various constraints. In short, Django Models is the SQL of Database one uses with Django. SQL (Structured Query Language) is complex and involves a lot of different queries for creating, deleting, updating or any other stuff related to database. Django models simplify the tasks and organize tables into models. Generally, each model maps to a single database table. This article revolves about how one can use Django models to store data in the database conveniently. Moreover, we can use admin panel of Django to create, update, delete or retrieve fields of a model and various similar operations. Django models provide simplicity, consistency, version control and advanced metadata handling. Basics of a model include –

• Each model is a Python class that subclasses django.db.models.Model.
• Each attribute of the model represents a database field.
• With all of this, Django gives you an automatically-generated database-access API; see Making queries.

from django.db import models 
  
# Create your models here. 
class McatutorialsModel(models.Model): 
    title = models.CharField(max_length = 200) 
    description = models.TextField()

Django maps the fields defined in Django models into table fields of the database as shown below.

To use Django Models, one needs to have a project and an app working in it. After you start an app you can create models in app/models.py. Before starting to use a model let’s check how to start a project and create an app named mcatutorials.py

Refer to the following articles to check how to create a project and an app in Django.

• How to Create a Basic Project using MVT in Django?
• How to Create an App in Django ?

from django.db import models
        
class ModelName(models.Model):
        field_name = models.Field(**options)

To create a model, in mcatutorials/models.py Enter the code,

# import the standard Django Model 
# from built-in library 
from django.db import models 
  
# declare a new model with a name "McatutorialsModel" 
class McatutorialsModel(models.Model): 
        # fields of the model 
    title = models.CharField(max_length = 200) 
    description = models.TextField() 
    last_modified = models.DateTimeField(auto_now_add = True) 
    img = models.ImageField(upload_to = "images/") 
  
        # renames the instances of the model 
        # with their title name 
    def __str__(self): 
        return self.title

Whenever we create a Model, Delete a Model, or update anything in any of models.py of our project. We need to run two commands makemigrations and migrate. makemigrations basically generates the SQL commands for preinstalled apps (which can be viewed in installed apps in settings.py) and your newly created app’s model which you add in installed apps whereas migrate executes those SQL commands in the database file. So when we run,

Python manage.py makemigrations

SQL Query to create above Model as a Table is created and

 Python manage.py migrate

creates the table in the database.

Now we have created a model we can perform various operations such as creating a Row for the table or in terms of Django Creating an instance of Model. To know more visit – Django Basic App Model – Makemigrations and Migrate

To render a model in Django admin, we need to modify app/admin.py. Go to admin.py in mcatutorials app and enter the following code. Import the corresponding model from models.py and register it to the admin interface.

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

Now we can check whether the model has been rendered in Django Admin. Django Admin Interface can be used to graphically implement CRUD (Create, Retrieve, Update, Delete).

To check more on rendering models in django admin, visit – Render Model in Django Admin Interface

Django lets us interact with its database models, i.e. add, delete, modify and query objects, using a database-abstraction API called ORM(Object Relational Mapper). We can access the Django ORM by running the following command inside our project directory.

python manage.py shell

To create an object of model Album and save it into the database, we need to write the following command:

>>> a = McatutorialsModel(
         title = “McatutorialsForMcatutorials”,  
         description = “A description here”,
         img = “Mcatutorials/abc.png”
         )
>>> a.save()

To retrieve all the objects of a model, we write the following command:

>>> McatutorialsModel.objects.all()
< QuerySet [< McatutorialsModel: Divide>, < McatutorialsModel: Abbey Road>, < McatutorialsModel: Revolver>]>

We can modify an existing object as follows:

>>> a = McatutorialsModel.objects.get(id = 3)
>>> a.title = "Pop"
>>> a.save()

To delete a single object, we need to write the following commands:

>>> a = Album.objects.get(id = 2)
>>> a.delete()

To check detailed post of Django’s ORM (Object) visit Django ORM – Inserting, Updating & Deleting Data

Built-in Field Validations in Django models are the default validations that come predefined to all Django fields. Every field comes in with built-in validations from Django validators. For example, IntegerField comes with built-in validation that it can only store integer values and that too in a particular range.
Enter the following code into models.py file of mcatutorials app.

from django.db import models 
from django.db.models import Model 
# Create your models here. 
  
class McatutorialsModel(Model): 
    mcatutorials_field = models.IntegerField() 
  
    def __str__(self): 
        return self.mcatutorials_field

After running makemigrations and migrate on Django and rendering above model, let us try to create an instance using string “GfG is Best“.

You can see in the admin interface, one can not enter a string in an IntegerField. Similarly every field has its own validations. To know more about validations visit, Built-in Field Validations – Django Models

• Change Object Display Name using __str__ function – Django Models
• Custom Field Validations in Django Models
• Django python manage.py migrate command
• Django App Model – Python manage.py makemigrations command
• Django model data types and fields list
• How to use Django Field Choices ?
• Overriding the save method – Django Models

The most important part of a model and the only required part of a model is the list of database fields it defines. Fields are specified by class attributes. Here is a list of all Field types used in Django.

Django also defines a set of fields that represent relations.

Field Options are the arguments given to each field for applying some constraint or imparting a particular characteristic to a particular Field. For example, adding an argument null = True to CharField will enable it to store empty values for that table in relational database. Here are the field options and attributes that an CharField can use.