"Save as new" breaks when related inlines are marked to be deleted

[Alex Marandon]

Minimal project that triggers the issue:

# model.py
from django.db import models


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

    def __unicode__(self):
        return self.name


class Book(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author)

    def __unicode__(self):
        return self.title

# admin.py
from django.contrib import admin
from .models import Author, Book


class AuthorshipInline(admin.TabularInline):
    model = Book.authors.through


@admin.register(Author)
class AuthorAdmin(admin.ModelAdmin):
    inlines = [AuthorshipInline]
    save_as = True


@admin.register(Book)
class BookAdmin(admin.ModelAdmin):
    inlines = [AuthorshipInline]
    save_as = True

How to reproduce the problem using the admin:

• create two authors
• create a book with those two authors
• edit the book
• click the check box in the "Delete?" column for one of the authors
• click "Save as new"

Tail of the stack trace:

File "/home/synext/lib/python2.7/site-packages/Django-1.7.1-py2.7.egg/django/db/models/deletion.py" in collect
  168.                             reverse_dependency=reverse_dependency)
File "/home/synext/lib/python2.7/site-packages/Django-1.7.1-py2.7.egg/django/db/models/deletion.py" in add
  85.             if obj not in instances:
File "/home/synext/lib/python2.7/site-packages/Django-1.7.1-py2.7.egg/django/db/models/base.py" in __hash__
  485.             raise TypeError("Model instances without primary key value are unhashable")

Exception Type: TypeError at /admin/myapp/book/1/
Exception Value: Model instances without primary key value are unhashable

[Tim Graham]

I could reproduce this by adding save_as=True to PollAdmin from the tutorial and then repeating the steps above. It appears to be a regression in 1.7, probably due to 6af05e7a0f0e4604d6a67899acaa99d73ec0dfaa.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In c7a19f42030c15ad3b3475ad9a4854e10733ff74:

Fixed #23857 -- Fixed admin crash with "save as new" and deleting inline.

Thanks amarandon for the report.

[Tim Graham <timograham@…>]

In c64286c62b5960387d5e8fc3e28cd37662bda6cb:

[1.7.x] Fixed #23857 -- Fixed admin crash with "save as new" and deleting inline.

Thanks amarandon for the report.

Backport of c7a19f42030c15ad3b3475ad9a4854e10733ff74 from master