Django on_delete – Advanced Django Concepts

The on_delete attribute is a parameter used in Django to specify what should happen when an item that is referenced by a ForeignKey is deleted.

Syntax

models.ForeignKey(to, on_delete)

Example

Let's consider a simple example of models.py with a foreign key relationship. Here is the code snippet:

class Country(models.Model):
    name = models.CharField(max_length=50, unique=True)

class City(models.Model):
    name = models.CharField(max_length=50)
    country = models.ForeignKey(Country, on_delete=models.CASCADE)

Here, City model has a foreign key relationship with Country using on_delete parameter.

Explanation

The on_delete attribute is mandatory for ForeignKey field. It specifies how to handle when a referenced object is deleted.

Here are the following possible values for on_delete:

• models.CASCADE: This value deletes the object containing the foreign key when the referenced object is deleted.
• models.PROTECT: This value prevents deletion of the referenced object when related objects still have references to it.
• models.SET_NULL: This value sets the foreign key to NULL when the referenced object is deleted. This requires the foreign key to have null=True.
• models.SET_DEFAULT: This value sets the foreign key to its default value when the referenced object is deleted.
• models.DO_NOTHING: This value does nothing.

Use

on_delete attribute is used in ForeignKey fields to specify the behavior that should occur when the referenced object is deleted.

Important Points

1. on_delete attribute is mandatory for the ForeignKey field.
2. Different values can be used for on_delete attribute.
3. Use CASCADE with caution, as it may cause unexpected delete on related objects.
4. It is a good practice to use on_delete=models.PROTECT for critical data relationships.

Summary

on_delete attribute is an important identifier used in Django to specify the behavior that should occur when the referenced object is deleted. It is a mandatory attribute for the ForeignKey field in Django. Various values of on_delete provide different behaviors to handle data referencing in Django. Always use appropriate on_delete behavior depending upon the project requirement.