{"id":5779,"date":"2022-11-24T08:14:55","date_gmt":"2022-11-24T08:14:55","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=5779"},"modified":"2023-08-17T08:56:01","modified_gmt":"2023-08-17T08:56:01","slug":"django-models","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/django-tutorial\/django-models\/","title":{"rendered":"Django Models"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about Django models and how to create models for your Django application.<\/p>\n\n\n\n

This tutorial begins where creating Django templates tutorial<\/a> left off.<\/p>\n\n\n\n

Introduction to Django models #<\/a><\/h2>\n\n\n\n

In Django, a model is a subclass of the django.db.models.Model<\/code> class. A model contains one or more fields and methods that manipulate the fields.<\/p>\n\n\n\n

Essentially, a Django model maps to a single table in the database in which each field of the model represents a column in the table.<\/p>\n\n\n\n

An application may have zero or more models stored in models.py<\/code> module. For example, the following defines a Post<\/code> model for the blog<\/code> application:<\/p>\n\n\n

from<\/span> django.db import<\/span> models\nfrom<\/span> django.utils import<\/span> timezone\n\n\nclass<\/span> Post<\/span>(models.Model)<\/span>:<\/span>\n    title = models.CharField(max_length=120<\/span>)\n    content = models.TextField()\n    published_at = models.DateTimeField(default=timezone.now)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The Post<\/code> model has the title<\/code>, content<\/code>, and published_at<\/code> fields. Based on the Post<\/code> model, Django will create a table in the database with the following SQL code:<\/p>\n\n\n
CREATE<\/span> TABLE<\/span> \"blog_post\"<\/span> (\n  \"id\"<\/span> integer<\/span> NOT<\/span> NULL<\/span> PRIMARY KEY<\/span> AUTOINCREMENT, \n  \"title\"<\/span> varchar<\/span>(120<\/span>) NOT<\/span> NULL<\/span>, \n  \"content\"<\/span> text<\/span> NOT<\/span> NULL<\/span>, \n  \"published_at\"<\/span> datetime NOT<\/span> NULL<\/span>, \n);<\/code><\/span>Code language:<\/span> SQL (Structured Query Language)<\/span> (<\/span>sql<\/span>)<\/span><\/small><\/pre>\n\n\n
The name of the table blog_post<\/code> is automatically derived from the application and model names:<\/p>\n\n\n
application.model<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, Django will create a table blog_post<\/code> for the Post<\/code> model.<\/p>\n\n\n\n
To specify a table name instead of using the default name generated by Django, you can use the db_table<\/code> attribute of the Meta<\/code> class like this:<\/p>\n\n\n
from<\/span> django.db import<\/span> models\n<\/span><\/span>from<\/span> django.utils import<\/span> timezone\n<\/span><\/span>\n<\/span><\/span>\n<\/span><\/span>class<\/span> Post<\/span>(models.Model)<\/span>:<\/span>\n<\/span><\/span>    title = models.CharField(max_length=120<\/span>)\n<\/span><\/span>    content = models.TextField()\n<\/span><\/span>    published_at = models.DateTimeField(default=timezone.now)\n<\/span><\/span>    \n<\/span><\/span>    class<\/span> Meta<\/span>:<\/span>\n<\/span><\/span>        db_table = 'posts'<\/span>\n<\/span><\/mark><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this case, the Post<\/code> model will map to the posts<\/code> table instead of the generated blog_post<\/code> table. In this tutorial, we’ll sticky with the default generated table name blog_post<\/code>.<\/p>\n\n\n\n
When creating a table, Django automatically adds the id<\/code> field as the primary key of the table. The id<\/code> field is an auto-increment field with the type specified in the settings.py<\/code> file of the project:<\/p>\n\n\n
DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you want to specify your own primary key field, you need to explicitly define it in the model like this:<\/p>\n\n\n
post_id = models.BigAutoField(primary_key=True<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Django requires each model to have exactly one field<\/strong> with the primary_key=True<\/code>.<\/p>\n\n\n\n
Using models #<\/a><\/h3>\n\n\n\n
Once defining models, you need to tell Django that you’re going to use them by registering the application name in the INSTALLED_APPS<\/code> list in the settings.py<\/code> of the project:<\/p>\n\n\n
INSTALLED_APPS = [\n    # ...<\/span>\n    'blog.apps.BlogConfig'<\/span>,\n]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Built-in models #<\/a><\/h3>\n\n\n\n
Django comes with some built-in models like User<\/code> from django.contrib.auth.models<\/code> module. To use the User<\/code> model, you need to import it into the models.py<\/code> file:<\/p>\n\n\n
from<\/span> django.contrib.auth.models import<\/span> User<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Foreign keys #<\/a><\/h3>\n\n\n\n
Each post in the blog<\/code> application is created by a user and a user may create zero or more posts. This is called a one-to-many relationship<\/a>. <\/p>\n\n\n\n
To model the one-to-many relationship, you use the ForeignKey<\/code> field:<\/p>\n\n\n
from<\/span> django.db import<\/span> models\n<\/span><\/span>from<\/span> django.utils import<\/span> timezone\n<\/span><\/span>from<\/span> django.contrib.auth.models import<\/span> User\n<\/span><\/span>\n<\/span><\/span>\n<\/span><\/span>class<\/span> Post<\/span>(models.Model)<\/span>:<\/span>\n<\/span><\/span>    title = models.CharField(max_length=120<\/span>)\n<\/span><\/span>    content = models.TextField()\n<\/span><\/span>    published_at = models.DateTimeField(default=timezone.now)\n<\/span><\/span>    author = models.ForeignKey(User, on_delete=models.CASCADE)\n<\/span><\/mark><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Based on this model, Django will create the blog_post<\/code> table with the following structure:<\/p>\n\n\n
CREATE<\/span> TABLE<\/span> \"blog_post\"<\/span> (\n  \"id\"<\/span> integer<\/span> NOT<\/span> NULL<\/span> PRIMARY KEY<\/span> AUTOINCREMENT, \n  \"title\"<\/span> varchar<\/span>(120<\/span>) NOT<\/span> NULL<\/span>, \n  \"content\"<\/span> text<\/span> NOT<\/span> NULL<\/span>, \n  \"published_at\"<\/span> datetime NOT<\/span> NULL<\/span>, \n  \"author_id\"<\/span> integer<\/span> NOT<\/span> NULL<\/span> \n   REFERENCES<\/span> \"auth_user\"<\/span> (\"id\"<\/span>) \n   DEFERRABLE INITIALLY<\/span> DEFERRED<\/span>\n);<\/code><\/span>Code language:<\/span> SQL (Structured Query Language)<\/span> (<\/span>sql<\/span>)<\/span><\/small><\/pre>\n\n\n
The __str__() method #<\/a><\/h3>\n\n\n\n
To define the string representation of a model, you can override the __str__()<\/code> method. For example:<\/p>\n\n\n
from<\/span> django.db import<\/span> models\nfrom<\/span> django.utils import<\/span> timezone\nfrom<\/span> django.contrib.auth.models import<\/span> User\n\n\nclass<\/span> Post<\/span>(models.Model)<\/span>:<\/span>\n    title = models.CharField(max_length=120<\/span>)\n    content = models.TextField()\n    published_at = models.DateTimeField(default=timezone.now)\n    author = models.ForeignKey(User, on_delete=models.CASCADE)\n    \n\n    def<\/span> __str__<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.title<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Adding Meta class to the Model class #<\/a><\/h3>\n\n\n\n
The Meta<\/code> class allows you to configure the model. For example, the following defines the Meta<\/code> class inside the Post<\/code> model class that sorts the posts by the published_at<\/code> in descending order (-published_at<\/code>) i.e., the newer posts first and the older posts after.<\/p>\n\n\n
from<\/span> django.db import<\/span> models\nfrom<\/span> django.utils import<\/span> timezone\nfrom<\/span> django.contrib.auth.models import<\/span> User\n\nclass<\/span> Post<\/span>(models.Model)<\/span>:<\/span>\n    title = models.CharField(max_length=120<\/span>)\n    content = models.TextField()\n    published_at = models.DateTimeField(default=timezone.now)\n    author = models.ForeignKey(User, on_delete=models.CASCADE)\n    \n\n    def<\/span> __str__<\/span>(self)<\/span>:<\/span>\n        return<\/span> self.title\n    \n    class<\/span> Meta<\/span>:<\/span>\n        ordering = ['-published_at'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
After defining models, you can create and apply migrations to create tables in the database, which we’ll cover in the next tutorial.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • Define all models in the models.py<\/code> file of the Django application.<\/li>\n\n\n\n
  • Define a class that inherits from the django.db.models.Model<\/code> to create a model.<\/li>\n\n\n\n
  • A model maps to a table in the database, in which each field maps to a column in the database table.<\/li>\n\n\n\n
  • Override __str__()<\/code> method to return a string representation of a model.<\/li>\n\n\n\n
  • Use the Meta<\/code> class to configure the model.<\/li>\n<\/ul>\n
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Download the Django Project source code<\/a><\/p>\n\n\n\n
When you use the instance of the Post model as a string, Django calls the __str__()<\/a><\/code> method and displays its result.<\/p>\n\n\n\n
In this example, the auth_id<\/code> is a foreign key<\/a> that creates a relationship between blog_post<\/code> table and auth_user<\/code> table. Note that the auth_user<\/code> table is the Django-provided table.<\/p>\n\n\n\n
In this example, the primary_key=True<\/code> indicates that the post_id<\/code> is a primary key<\/a>. When Django sees a field in the model with the primary_key=True<\/code>, it won’t add the automatic id<\/code> column.<\/p>\n\n\n\n
Note that the above-generated SQL<\/a> is for SQLite<\/a>. If you use a different database, you’ll see that the SQL code is slightly different.<\/p>\n\n\n\n