What does on_delete do on Django models?

This is the behaviour to adopt when the referenced object is deleted. It is not specific to Django; this is an SQL standard. Although Django has its own implementation on top of SQL. (1)

There are seven possible actions to take when such event occurs:

CASCADE: When the referenced object is deleted, also delete the objects that have references to it (when you remove a blog post for instance, you might want to delete comments as well). SQL equivalent: CASCADE.

PROTECT: Forbid the deletion of the referenced object. To delete it you will have to delete all objects that reference it manually. SQL equivalent: RESTRICT.

RESTRICT: (introduced in Django 3.1) Similar behavior as PROTECT that matches SQL's RESTRICT more accurately. (See django documentation example)

SET_NULL: Set the reference to NULL (requires the field to be nullable). For instance, when you delete a User, you might want to keep the comments he posted on blog posts, but say it was posted by an anonymous (or deleted) user. SQL equivalent: SET NULL.

SET_DEFAULT: Set the default value. SQL equivalent: SET DEFAULT.

SET(...): Set a given value. This one is not part of the SQL standard and is entirely handled by Django.

DO_NOTHING: Probably a very bad idea since this would create integrity issues in your database (referencing an object that actually doesn't exist). SQL equivalent: NO ACTION. (2)

Source: Django documentation

See also the documentation of PostgreSQL for instance.

In most cases, CASCADE is the expected behaviour, but for every ForeignKey, you should always ask yourself what is the expected behaviour in this situation. PROTECT and SET_NULL are often useful. Setting CASCADE where it should not, can potentially delete all of your database in cascade, by simply deleting a single user.

Additional note to clarify cascade direction

It's funny to notice that the direction of the CASCADE action is not clear to many people. Actually, it's funny to notice that only the CASCADE action is not clear. I understand the cascade behavior might be confusing, however you must think that it is the same direction as any other action. Thus, if you feel that CASCADE direction is not clear to you, it actually means that on_delete behavior is not clear to you.

In your database, a foreign key is basically represented by an integer field which value is the primary key of the foreign object. Let's say you have an entry comment_A, which has a foreign key to an entry article_B. If you delete the entry comment_A, everything is fine. article_B used to live without comment_A and don't bother if it's deleted. However, if you delete article_B, then comment_A panics! It never lived without article_B and needs it, and it's part of its attributes (article=article_B, but what is article_B???). This is where on_delete steps in, to determine how to resolve this integrity error, either by saying:

"No! Please! Don't! I can't live without you!" (which is said PROTECT or RESTRICT in Django/SQL)

"All right, if I'm not yours, then I'm nobody's" (which is said SET_NULL)

"Good bye world, I can't live without article_B" and commit suicide (this is the CASCADE behavior).

"It's OK, I've got spare lover, and I'll reference article_C from now" (SET_DEFAULT, or even SET(...)).

"I can't face reality, and I'll keep calling your name even if that's the only thing left to me!" (DO_NOTHING)

I hope it makes cascade direction clearer. :)

Footnotes

(1) Django has its own implementation on top of SQL. And, as mentioned by @JoeMjr2 in the comments below, Django will not create the SQL constraints. If you want the constraints to be ensured by your database (for instance, if your database is used by another application, or if you hang in the database console from time to time), you might want to set the related constraints manually yourself. There is an open ticket to add support for database-level on delete constrains in Django.

(2) Actually, there is one case where DO_NOTHING can be useful: If you want to skip Django's implementation and implement the constraint yourself at the database-level.

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The on_delete method is used to tell Django what to do with model instances that depend on the model instance you delete. (e.g. a ForeignKey relationship). The on_delete=models.CASCADE tells Django to cascade the deleting effect i.e. continue deleting the dependent models as well.

Here's a more concrete example. Assume you have an Author model that is a ForeignKey in a Book model. Now, if you delete an instance of the Author model, Django would not know what to do with instances of the Book model that depend on that instance of Author model. The on_delete method tells Django what to do in that case. Setting on_delete=models.CASCADE will instruct Django to cascade the deleting effect i.e. delete all the Book model instances that depend on the Author model instance you deleted.

Note: on_delete will become a required argument in Django 2.0. In older versions it defaults to CASCADE.

Here's the entire official documentation.

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FYI, the on_delete parameter in models is backwards from what it sounds like. You put on_delete on a foreign key (FK) on a model to tell Django what to do if the FK entry that you are pointing to on your record is deleted. The options our shop have used the most are PROTECT, CASCADE, and SET_NULL. Here are the basic rules I have figured out:

Use PROTECT when your FK is pointing to a look-up table that really shouldn't be changing and that certainly should not cause your table to change. If anyone tries to delete an entry on that look-up table, PROTECT prevents them from deleting it if it is tied to any records. It also prevents Django from deleting your record just because it deleted an entry on a look-up table. This last part is critical. If someone were to delete the gender "Female" from my Gender table, I CERTAINLY would NOT want that to instantly delete any and all people I had in my Person table who had that gender. Use CASCADE when your FK is pointing to a "parent" record. So, if a Person can have many PersonEthnicity entries (he/she can be American Indian, Black, and White), and that Person is deleted, I really would want any "child" PersonEthnicity entries to be deleted. They are irrelevant without the Person. Use SET_NULL when you do want people to be allowed to delete an entry on a look-up table, but you still want to preserve your record. For example, if a Person can have a HighSchool, but it doesn't really matter to me if that high-school goes away on my look-up table, I would say on_delete=SET_NULL. This would leave my Person record out there; it just would just set the high-school FK on my Person to null. Obviously, you will have to allow null=True on that FK.

Here is an example of a model that does all three things:

class PurchPurchaseAccount(models.Model):
    id = models.AutoField(primary_key=True)
    purchase = models.ForeignKey(PurchPurchase, null=True, db_column='purchase', blank=True, on_delete=models.CASCADE) # If "parent" rec gone, delete "child" rec!!!
    paid_from_acct = models.ForeignKey(PurchPaidFromAcct, null=True, db_column='paid_from_acct', blank=True, on_delete=models.PROTECT) # Disallow lookup deletion & do not delete this rec.
    _updated = models.DateTimeField()
    _updatedby = models.ForeignKey(Person, null=True, db_column='_updatedby', blank=True, related_name='acctupdated_by', on_delete=models.SET_NULL) # Person records shouldn't be deleted, but if they are, preserve this PurchPurchaseAccount entry, and just set this person to null.

    def __unicode__(self):
        return str(self.paid_from_acct.display)
    class Meta:
        db_table = u'purch_purchase_account'

As a last tidbit, did you know that if you don't specify on_delete (or didn't), the default behavior is CASCADE? This means that if someone deleted a gender entry on your Gender table, any Person records with that gender were also deleted!

I would say, "If in doubt, set on_delete=models.PROTECT." Then go test your application. You will quickly figure out which FKs should be labeled the other values without endangering any of your data.

Also, it is worth noting that on_delete=CASCADE is actually not added to any of your migrations, if that is the behavior you are selecting. I guess this is because it is the default, so putting on_delete=CASCADE is the same thing as putting nothing.

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As mentioned earlier, CASCADE will delete the record that has a foreign key and references another object that was deleted. So for example if you have a real estate website and have a Property that references a City

class City(models.Model):
    # define model fields for a city

class Property(models.Model):
    city = models.ForeignKey(City, on_delete = models.CASCADE)
    # define model fields for a property

and now when the City is deleted from the database, all associated Properties (eg. real estate located in that city) will also be deleted from the database

Now I also want to mention the merit of other options, such as SET_NULL or SET_DEFAULT or even DO_NOTHING. Basically, from the administration perspective, you want to "delete" those records. But you don't really want them to disappear. For many reasons. Someone might have deleted it accidentally, or for auditing and monitoring. And plain reporting. So it can be a way to "disconnect" the property from a City. Again, it will depend on how your application is written.

For example, some applications have a field "deleted" which is 0 or 1. And all their searches and list views etc, anything that can appear in reports or anywhere the user can access it from the front end, exclude anything that is deleted == 1. However, if you create a custom report or a custom query to pull down a list of records that were deleted and even more so to see when it was last modified (another field) and by whom (i.e. who deleted it and when)..that is very advantageous from the executive standpoint.

And don't forget that you can revert accidental deletions as simple as deleted = 0 for those records.

My point is, if there is a functionality, there is always a reason behind it. Not always a good reason. But a reason. And often a good one too.

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Using CASCADE means actually telling Django to delete the referenced record. In the poll app example below: When a 'Question' gets deleted it will also delete the Choices this Question has.

e.g Question: How did you hear about us? (Choices: 1. Friends 2. TV Ad 3. Search Engine 4. Email Promotion)

When you delete this question, it will also delete all these four choices from the table. Note that which direction it flows. You don't have to put on_delete=models.CASCADE in Question Model put it in the Choice.

from django.db import models

class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.dateTimeField('date_published')

class Choice(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    choice_text = models.CharField(max_legth=200)
    votes = models.IntegerField(default=0)

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simply put, on_delete is an instruction to specify what modifications will be made to the object in case the foreign object is deleted:

CASCADE: will remove the child object when the foreign object is deleted

SET_NULL: will set the child object foreign key to null

SET_DEFAULT: will set the child object to the default data given while creating the model

RESTRICT: raises a RestrictedError under certain conditions.

PROTECT: prevents the foreign object from being deleted so long there are child objects inheriting from it

additional links:

https://docs.djangoproject.com/en/4.0/ref/models/fields/#foreignkey

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Here is answer for your question that says: why we use on_delete?

When an object referenced by a ForeignKey is deleted, Django by default emulates the behavior of the SQL constraint ON DELETE CASCADE and also deletes the object containing the ForeignKey. This behavior can be overridden by specifying the on_delete argument. For example, if you have a nullable ForeignKey and you want it to be set null when the referenced object is deleted:

user = models.ForeignKey(User, blank=True, null=True, on_delete=models.SET_NULL)

The possible values for on_delete are found in django.db.models:

CASCADE: Cascade deletes; the default.

PROTECT: Prevent deletion of the referenced object by raising ProtectedError, a subclass of django.db.IntegrityError.

SET_NULL: Set the ForeignKey null; this is only possible if null is True.

SET_DEFAULT: Set the ForeignKey to its default value; a default for the ForeignKey must be set.

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Let's say you have two models, one named Person and another one named Companies.

By definition, one person can create more than one company.

Considering a company can have one and only one person, we want that when a person is deleted that all the companies associated with that person also be deleted.

So, we start by creating a Person model, like this

class Person(models.Model):
    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.id+self.name

Then, the Companies model can look like this

class Companies(models.Model):
    title = models.CharField(max_length=20)
    description=models.CharField(max_length=10)
    person= models.ForeignKey(Person,related_name='persons',on_delete=models.CASCADE)

Notice the usage of on_delete=models.CASCADE in the model Companies. That is to delete all companies when the person that owns it (instance of class Person) is deleted.

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Reorient your mental model of the functionality of "CASCADE" by thinking of adding a FK to an already existing cascade (i.e. a waterfall). The source of this waterfall is a primary key (PK). Deletes flow down.

So if you define a FK's on_delete as "CASCADE," you're adding this FK's record to a cascade of deletes originating from the PK. The FK's record may participate in this cascade or not ("SET_NULL"). In fact, a record with a FK may even prevent the flow of the deletes! Build a dam with "PROTECT."

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Deletes all child fields in the database when parent object is deleted then we use on_delete as so:

class user(models.Model):
    commodities = models.ForeignKey(commodity, on_delete=models.CASCADE)

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CASCADE will also delete the corresponding field connected with it.

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