Django - Cascade deletion in ManyToManyRelation

Question

Using the following related models (one blog entry can have multiple revisions):

class BlogEntryRevision(models.Model):
    revisionNumber = models.IntegerField()
    title = models.CharField(max_length = 120)
    text = models.TextField()
    [...]

class BlogEntry(models.Model):
    revisions = models.ManyToManyField(BlogEntryRevision)
    [...]

How can I tell Django to delete all related BlogEntryRevisions when the corresponding BlogEntry is deleted? The default seems to be to keep objects in a many-to-many relation if an object of the "other" side is deleted. Any way to do this - preferably without overriding BlogEntry.delete?

Answer 1

I think you are misunderstanding the nature of a ManyToMany relationship. You talk about "the corresponding BlogEntry" being deleted. But the whole point of a ManyToMany is that each BlogEntryRevision has multiple BlogEntries related to it. (And, of course, each BlogEntry has multiple BlogEntryRevisions, but you know that already.)

From the names you have used, and the fact that you want this deletion cascade functionality, I think you would be better off with a standard ForeignKey from BlogEntryRevision to BlogEntry. As long as you don't set null=True on that ForeignKey, deletions will cascade - when the BlogEntry is deleted, all Revisions will be too.

As Of Django 2.0

The ForeignKey initializer now requires you to specify the on_delete parameter:

from django.db import models
from .models import MyRelatedModel


class model(models.Model):
    related_model = models.ForeignKey(MyRelatedModel, on_delete=models.CASCADE)

Answer 2

I had this exact use-case today:

• Model Author: can have several entries
• Model Entry: can have several authors

For this, I'm using a ManyToManyRelationship.

My use-case was: if I delete the last entry of a particular author, then this author should be deleted as well.

The solution can be achieved using the pre_delete signal:

@receiver(pre_delete, sender=Entry)
def pre_delete_story(sender, instance, **kwargs):
    for author in instance.authors.all():
        if author.entries.count() == 1 and instance in author.entries.all():
            # instance is the only Entry authored by this Author, so delete it
            author.delete()

Answer 3

Simply use the clear() method to remove related objects since Django uses a through model to specify the relationships the clear method removes all related BlogEntryRevision

be = BlogEntry.objects.get(id=1)
be.blogentryrevision_set.clear()

Answer 4

You can use a custom model manager, but the documentation seems to indicate that it does do something like this already and I can't recall exactly what this means:

The delete method, conveniently, is named delete(). This method immediately deletes the object and has no return value. Example:

e.delete()

You can also delete objects in bulk. Every QuerySet has a delete() method, which deletes all members of that QuerySet.

For example, this deletes all Entry objects with a pub_date year of 2005:

Entry.objects.filter(pub_date__year=2005).delete()

Keep in mind that this will, whenever possible, be executed purely in SQL, and so the delete() methods of individual object instances will not necessarily be called during the process. If you've provided a custom delete() method on a model class and want to ensure that it is called, you will need to "manually" delete instances of that model (e.g., by iterating over a QuerySet and calling delete() on each object individually) rather than using the bulk delete() method of a QuerySet.

When Django deletes an object, it emulates the behavior of the SQL constraint ON DELETE CASCADE -- in other words, any objects which had foreign keys pointing at the object to be deleted will be deleted along with it. For example:

b = Blog.objects.get(pk=1)
# This will delete the Blog and all of its Entry objects.
b.delete()