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Django Models

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  • Difficulty Level : Medium
  • Last Updated : 11 Apr, 2022
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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.

Example – 
 

Python3




from django.db import models
 
# Create your models here.
class GeeksModel(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. 
 

django-models

 

Using Django Models

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 geeks.py
 

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

 

Creating a Model

Syntax 
 

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

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

Python3




# import the standard Django Model
# from built-in library
from django.db import models
 
# declare a new model with a name "GeeksModel"
class GeeksModel(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 
 

Render a model in Django Admin Interface

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

Python3




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

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). 
 

Django-Models-render-admin

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

Django CRUD – Inserting, Updating and Deleting Data

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

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

>>>> a = GeeksModel(

         title = "GeeksForGeeks",  

         description = "A description here",

         img = "geeks/abc.png"

         )

>>> a.save()

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

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

Modifying existing objects 
We can modify an existing object as follows:
 

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

Deleting objects 
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
 

Validation on Fields in a Model

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 geeks app. 
 

Python3




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

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

built-in-validation-django-models

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
 

More on Django Models –

 

 

Basic model data types and fields list

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. 
 

 

Field NameDescription
AutoFieldIt An IntegerField that automatically increments.
BigAutoFieldIt is a 64-bit integer, much like an AutoField except that it is guaranteed to fit numbers from 1 to 9223372036854775807.
BigIntegerFieldIt is a 64-bit integer, much like an IntegerField except that it is guaranteed to fit numbers from -9223372036854775808 to 9223372036854775807.
BinaryFieldA field to store raw binary data.
BooleanFieldA true/false field. 
The default form widget for this field is a CheckboxInput.
CharFieldIt is string filed for small to large-sized input
DateFieldA date, represented in Python by a datetime.date instance
 It is used for date and time, represented in Python by a datetime.datetime instance.
DecimalFieldIt is a fixed-precision decimal number, represented in Python by a Decimal instance.
DurationFieldA field for storing periods of time.
EmailFieldIt is a CharField that checks that the value is a valid email address.
FileFieldIt is a file-upload field.
FloatFieldIt is a floating-point number represented in Python by a float instance.
ImageFieldIt inherits all attributes and methods from FileField, but also validates that the uploaded object is a valid image.
IntegerFieldIt is an integer field. Values from -2147483648 to 2147483647 are safe in all databases supported by Django.
GenericIPAddressFieldAn IPv4 or IPv6 address, in string format (e.g. 192.0.2.30 or 2a02:42fe::4).
NullBooleanFieldLike a BooleanField, but allows NULL as one of the options.
PositiveIntegerFieldLike an IntegerField, but must be either positive or zero (0).
PositiveSmallIntegerFieldLike a PositiveIntegerField, but only allows values under a certain (database-dependent) point.
SlugFieldSlug is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They’re generally used in URLs.
SmallIntegerFieldIt is like an IntegerField, but only allows values under a certain (database-dependent) point.
TextFieldA large text field. The default form widget for this field is a Textarea.
TimeFieldA time, represented in Python by a datetime.time instance.
URLFieldA CharField for a URL, validated by URLValidator.
UUIDFieldA field for storing universally unique identifiers. Uses Python’s UUID class. When used on PostgreSQL, this stores in a uuid datatype, otherwise in a char(32).

 

Relationship Fields

Django also defines a set of fields that represent relations.

 

Field NameDescription
ForeignKeyA many-to-one relationship. Requires two positional arguments: the class to which the model is related and the on_delete option.
ManyToManyFieldA many-to-many relationship. Requires a positional argument: the class to which the model is related, which works exactly the same as it does for ForeignKey, including recursive and lazy relationships.
OneToOneFieldA one-to-one relationship. Conceptually, this is similar to a ForeignKey with unique=True, but the “reverse” side of the relation will directly return a single object.

 

Field Options

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.

 

Field OptionsDescription
NullIf True, Django will store empty values as NULL in the database. Default is False.
BlankIf True, the field is allowed to be blank. Default is False.
db_columnThe name of the database column to use for this field. If this isn’t given, Django will use the field’s name. 
 
DefaultThe default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created. 
 
help_textExtra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form. 
 
primary_keyIf True, this field is the primary key for the model.
editableIf False, the field will not be displayed in the admin or any other ModelForm. They are also skipped during model validation. Default is True
 
error_messagesThe error_messages argument lets you override the default messages that the field will raise. Pass in a dictionary with keys matching the error messages you want to override. 
 
help_textExtra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form. 
 
verbose_nameA human-readable name for the field. If the verbose name isn’t given, Django will automatically create it using the field’s attribute name, converting underscores to spaces. 
 
validatorsA list of validators to run for this field. See the validators documentation for more information. 
 
UniqueIf True, this field must be unique throughout the table. 
 

 


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