What is on_delete in Django?

When you create a ForeignKey, you must tell Django:

➡️ What should happen to the child object if the parent object is deleted?

Example:

student = models.ForeignKey(Student, on_delete=WHAT_HAPPENS_HERE) 

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✅ 1. models.CASCADE

Meaning:

If the parent is deleted → delete the child too.

Example:

If Student Rafi is deleted → all of Rafi's Results are also deleted.

When to use?

✔ When child data makes no sense without parent
✔ Example: Results cannot exist without a student

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✅ 2. models.SET_NULL

Meaning:

If parent is deleted → set the FK to NULL.

Example

class Student(models.Model):
    name = models.CharField(max_length=100)

class Result(models.Model):
    student = models.ForeignKey(Student, on_delete=models.SET_NULL)

If Student Rafi is deleted → Result records will stay, but:

student = NULL 

When to use?

✔ When child can exist without parent
✔ Example: Blog post author deleted → post remains

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✅ 3. models.PROTECT

Meaning:

Prevent deletion of parent if child exists.

Example:

If you try to delete a Student who has results:

❌ Django throws an error: ProtectedError
✔ This stops accidental data loss

When to use?

✔ Very important data
✔ Example: Cannot delete Category that still has products

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✅ 4. models.SET_DEFAULT

Meaning:

If parent is deleted → set FK to a default value.

Example:

If the linked student is deleted → student will become ID = 1.

When to use?

✔ Use when you want a fallback
✔ Example: Deleted author → assign to "Unknown Author"

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✅ 5. models.DO_NOTHING

Meaning:

Do nothing.
Django won't touch child objects.

BUT ⚠️
If DB has restrictions, you get errors.

Example:

on_delete=models.DO_NOTHING 

When to use?

❌ Almost never recommended
✔ Only when you handle deletion manually

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✅ 6. models.RESTRICT (Django 3.1+)

Meaning:

Like PROTECT, but safer and more predictable.

If child exists → prevent parent deletion.

Example:

student = models.ForeignKey(Student, on_delete=models.RESTRICT) 

When to use?

✔ Use instead of PROTECT
✔ Prevents accidental deletion cleanly

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🎯 Summary (Easy to Remember)