What Does `on_delete` do in Django Models?

Naveera A.

The Problem

What does the on_delete option do in Django models?

The Solution

The on_delete option determines how the deletion of referenced data will be handled by the database to maintain data integrity.

Let’s say we are making a learning management system (LMS). We define the following models:

class Student(models.Model):
    name = models.CharField(max_length = 255)

class Homework(models.Model):
    title = models.CharField(max_length = 255)
    student = models.ForeignKey(Student, on_delete=models.CASCADE)
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The Homework model stores a reference to the student who submitted that homework as a ForeignKey. A ForeignKey is a many-to-one relationship, which means that every student can submit multiple homework assignments, but each homework assignment can be submitted by only one student.

Let’s create an instance of both objects:

# Run these commands in Django shell
student = Student.objects.create(name="Jane")
homework = Homework.objects.create(title="Jane's homework", student=student_1)
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What happens if we want to delete homework? We can do so without any problem as student doesn’t contain any reference to homework. The student instance existed before we created homework and it can carry on existing after we delete homework.

But before we can delete student, we have to tell Django what to do with homework. Do we want orphaned homework instances lying around in our LMS? Or do we need to delete all the homework submitted by a student when that student is deleted?

This is where the on_delete option is used. It tells Django what behavior to adopt when the referenced object is deleted. We can set on_delete to one of seven options. Let’s take a look at each option:

CASCADE

Cascades deletes. When we delete student, homework will also be deleted.

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.CASCADE)


# Django shell
student.delete()

# also deletes homework
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PROTECT

Prevents deletion of the referenced object. When we try to delete student, we will get a ProtectedError:

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.PROTECT)


# Django shell
student.delete()

# Raises ProtectedError. We first need to delete homework before Django will allow us to delete student.

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RESTRICT

Prevents deletion of the referenced object. When we try to delete student, we will get a RestrictedError:

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.RESTRICT)


# Django shell
student.delete()

# Raises RestrictedError.

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Unlike PREVENT, RESTRICT allows deletion of a referenced object if it also references a different object that is being deleted in the same operation, but via a CASCADE relationship. To learn more, see the example in Django documentation.

SET_NULL

This option sets the ForeignKey to null, but we have to specify null=True in our model.

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.SET_NULL, null=True)


# Django shell
student.delete()

# Sets the foreign key in homework to null and deletes student.

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SET_DEFAULT

This option sets the ForeignKey to its default value, but we have to specify a default value in our model.

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.SET_DEFAULT, default=1)


# Django shell
student.delete()

# Sets the foreign key in homework to the student whose id is 1, and deletes student.

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SET()

This option sets the ForeignKey to the value passed to SET()

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.SET(get_student))


# Django shell
student.delete()

# Calls the get_student function that you have defined, sets the foreign key in homework to the value returned by the get_student function, and deletes student.

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DO_NOTHING

This option makes no changes to the object with the ForeignKey. In most cases, DO_NOTHING is a bad choice as it can create integrity issues. But some advance level use cases might need this option.

# models.py
class Homework(models.Model):
    ...
    student = models.ForeignKey(Student, on_delete=models.DO_NOTHING)


# Django shell
student.delete()

# Deletes student. Makes no changes to homework.

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