Django List objects that will be deleted by deleting the current object

Question

In my DeleteView template, I'd like to warn the user that deleting the current object will delete the following objects (since the db schema requires on_delete = models.CASCADE).

Here is an example model:

class Book(models.Model):
    course = models.ForeignKey(Course, related_name = 'course_books', on_delete = models.CASCADE, null = True, blank = True)

class Exercise(models.Model):
    book = models.ForeignKey(Book, related_name = 'exercises', on_delete = models.CASCADE, null = True, blank = True)
    ...

class Solution(models.Model):
    exercise = models.ForeignKey(Exercise, related_name = 'solutions', on_delete = models.CASCADE, null = True, blank = True)

So, say the user wants to delete a course, I'd like to warn the user which books, exercises, and solutions will be deleted upon deleting the current book. Something to this effect:

WARNING: Deleting course Programming 101 will delete Python Programming for Beginners and Python By Example.  It will also delete Excersises 1-5 and solutions 1-10

This will require that I can look at the related field to see if the on_delete is set to CASCADE or not. If it is, I should add it to the list of to_be_deleted objects.

I have looked to see if I can check the on_delete attribute of the field:

for related_object in the_course._meta.related_objects:
    dir(related_object.field)

But nothing shows up to help so I'm reaching out here...

score 7 · Answer 1 · answered Oct 02 '19 at 15:08

7

You can do so doing

from django.contrib.admin.utils import NestedObjects
from django.db import DEFAULT_DB_ALIAS

collector = NestedObjects(using=DEFAULT_DB_ALIAS)
collector.collect([obj])
print(collector.nested())

answered Oct 02 '19 at 15:08

Julien Kieffer

1,116
6
16

I had to move to another part of the project. I am revisiting the DeleteView and ran your code. When I type `the_project = Project.objects.filter(name__icontains = 'some_name')` I get `AttributeError: 'QuerySet' object has no attribute '_meta'` I did run `help(collector.collect)`, and it does say that I have to pass QuerySet to it, but I still get the error – EarlyCoder Dec 21 '19 at 22:20

score 0 · Answer 2 · answered Oct 02 '19 at 15:06

0

You can filter the objects with that particular object_id. For example:

qs = Solution.objects.filter(exercise__book__course_id=course_id).select_related('exercise__book')

This will give a queryset of all the objects of Solution with related foreign key objects exercise and book. Then you can loop through this queryset and check instance._meta.app_label to differentiate between the objects.

answered Oct 02 '19 at 15:06

Nalin Dobhal

2,292
2
10
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This solution works but it is a way around solution because you would have to know the related object and do all the work by hand. On a site where the relationships are across multiple apps, it is not practical. So, your solution is much appreciated and helpful, but I will wait for another solution for now – EarlyCoder Oct 02 '19 at 21:06
ok then you should edit your question so that it will be active again, otherwise it won't get enough attention. – Nalin Dobhal Oct 03 '19 at 06:53

score 0 · Answer 3 · answered Dec 09 '22 at 07:34

Maybe this is a duplicated question with How can I check what objects will be cascade deleted in Django?. So I paste the same answer:

I suggest using the NestedObjects util provided in django admin.

from django.contrib.admin.utils import NestedObjects
from django.db import router

using = router.db_for_write(Item._meta.model)
# if you only have one database, just set using = "default"

nested_object = NestedObjects(using)
nested_object.collect([Item])
# If you want to delete multi item, you can use:
# nested_object.collect(Model.objects.filter(type="deleted"))

print(nested_object.nested()

The result look like this:

Django List objects that will be deleted by deleting the current object

3 Answers3