If you are a Django developer, you may have encountered the following error message while working with models:
TypeError: __init__() missing 1 required positional argument: 'on_delete'
This error message occurs when you try to create a ForeignKey field in a Django model without specifying the on_delete argument. In this guide, we will explain what this error means, how to fix it, and how to avoid it in the future.
What does the error message mean?
When you create a ForeignKey field in a Django model, you need to specify the on_delete argument. This argument tells Django what to do when the referenced object is deleted. If you do not specify on_delete, you will get the TypeError error message.
How to fix the error
To fix the TypeError error, you need to add the on_delete argument to your ForeignKey field. Here is an example:
from django.db import models
class Author(models.Model):
name = models.CharField(max_length=100)
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.ForeignKey(Author, on_delete=models.CASCADE)
In this example, we have added the on_delete=models.CASCADE argument to the author field. This argument tells Django to delete the Book object when the associated Author object is deleted.
How to avoid the error
To avoid the TypeError error in the future, always specify the on_delete argument when creating a ForeignKey field. Here are the available options for the on_delete argument:
models.CASCADE: Cascade deletes. Django deletes the object containing the ForeignKey and all objects related to that object.models.PROTECT: Prevent deletion of the referenced object by raising a ProtectedError.models.SET_NULL: Set the ForeignKey to NULL.models.SET_DEFAULT: Set the ForeignKey to its default value.models.SET(): Set the ForeignKey to the value passed to SET().
FAQ
What is a ForeignKey field in Django?
A ForeignKey field is a field in a Django model that creates a relationship between two models. It is used to link one model to another model.
Why do I need to specify the on_delete argument in a ForeignKey field?
The on_delete argument tells Django what to do when the referenced object is deleted. If you do not specify on_delete, you will get a TypeError.
What does models.CASCADE do?
models.CASCADE is an option for the on_delete argument. It tells Django to delete the object containing the ForeignKey and all objects related to that object.
What is models.PROTECT?
models.PROTECT is an option for the on_delete argument. It prevents deletion of the referenced object by raising a ProtectedError.
What is models.SET_NULL?
models.SET_NULL is an option for the on_delete argument. It sets the ForeignKey to NULL when the referenced object is deleted.