prob. with non-related objects deleted

[snea...@gmx.net]

hello all,

my problem is the following (the models.py is attached):
i want to implement a double-linked list (type BasicEntry) with the first entry having an empty name. each entry has two attributes prev_entry and next_entry to its neigbours. the first entry is created automaticaly and should never be removed. upon removing the second to last entry (using the function removeEntry), the last entry is also removed, although i do not know why. i tracked it down to the "collect" function in "deletion.py" on line 190:

for related in model._meta.get_all_related_objects(
include_hidden=True, include_proxy_eq=True):

do i misunderstand django or is this a known problem or "feature"? although i think i removed all references from the to be removed entry to the other entries...


class BasicEntry(models.Model):
name = models.CharField(max_length=10)
entry_col = models.ForeignKey(EntryCollection)
next_entry = models.ForeignKey('BasicEntry', related_name='next', null=True, default=None)
prev_entry = models.ForeignKey('BasicEntry', related_name='prev', null=True, default=None)

def addEntry(self, name):
try_nr = 1
new_entry = None
while True:
try:
with transaction.atomic():
new_entry = BasicEntry(name=name,
entry_col=self.entry_col,
next_entry=self.next_entry,
prev_entry=self)
new_entry.save()
self.next_entry.prev_entry = new_entry
self.next_entry.save()
self.next_entry = new_entry
if self.prev_entry == self:
self.prev_entry = new_entry
self.save()
except IntegrityError as e:
print('addEntry: try %d' %(try_nr))
try_nr += 1
if try_nr >= DB_Transaction_Tries:
raise(e)
time.sleep(DB_Transaction_sleep)
else:
break
return(new_entry)

def removeEntry(self):
if self.name == '':
print('** The "virtual" first track can not be removed!')
return
try_nr = 1
while True:
try:
with transaction.atomic():
self.prev_entry.next_entry = self.next_entry
self.prev_entry.save()
self.next_entry.prev_entry = self.prev_entry
self.next_entry.save()
self.next_entry = None
self.prev_entry = None
self.save()
self.delete()
except IntegrityError as e:
print('removeEntry: try %d' %(try_nr))
try_nr += 1
if try_nr >= DB_Transaction_Tries:
raise(e)
time.sleep(DB_Transaction_sleep)
else:
break



a few details:
linux (debian amd64)
python3 (v3.4.2-2)
django (v1.7.1-1)

here's an example in postgres:

django=> select * from entry_test_basicentry order by id;
id | name | entry_col_id | next_entry_id | prev_entry_id
----+-------+--------------+---------------+---------------
33 | | 14 | 34 | 35
34 | test1 | 14 | 35 | 33
35 | test2 | 14 | 33 | 34
(3 rows)


after removing ID 34:

django=> select * from entry_test_basicentry order by id;
id | name | entry_col_id | next_entry_id | prev_entry_id
----+-------+--------------+---------------+---------------
33 | | 14 | 35 | 35
35 | test2 | 14 | 33 | 33
(2 rows)


after removing ID 35:

django=> select * from entry_test_basicentry order by id;
id | name | entry_col_id | next_entry_id | prev_entry_id
----+------+--------------+---------------+---------------
(0 rows)


thanks in advance and best regards


tom

[James Schneider]

This looks a lot like a basic MPTT tree. Have you looked at django-mptt or django-treebeard? Either of those packages could probably save you a ton of headache here, assuming that your lists contain all of the same type of model, which it appears they do.

http://django-mptt.github.io/django-mptt/overview.html#what-is-modified-preorder-tree-traversal

https://tabo.pe/projects/django-treebeard/docs/tip/

-James

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[snea...@gmx.net]

thank you for your answer, james.

 

i had a quick look, but unfortunatly, both are not readily available as .deb packages for python3, so i just just had some tries and came away with the following solution. in the previously mentioned "removeEntry" method, i changed the "with" block to the following:

 

                with transaction.atomic():
                    if self.prev_entry == self.next_entry:
                        self.prev_entry.next_entry = self.prev_entry
                        self.prev_entry.prev_entry = self.prev_entry
                        self.prev_entry.save()
                    else:

                        self.prev_entry.next_entry = self.next_entry
                        self.prev_entry.save()
                        self.next_entry.prev_entry = self.prev_entry
                        self.next_entry.save()
                    self.next_entry = None
                    self.prev_entry = None
                    self.save()
                    self.delete()

 

now it works as it should. i have a feeling that in the previous version, when there where only 2 entries left, self.prev_entry.next_entry and self.next_entry.prev_entry were different objects in django, although they are the same database objects.

 

best regards

 

 

tom

 

 

Gesendet: Dienstag, 06. Januar 2015 um 12:56 Uhr
Von: "James Schneider" <jrschn...@gmail.com>
An: django...@googlegroups.com
Betreff: Re: prob. with non-related objects deleted

To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/CA%2Be%2BciWtP93C21m57dSi8c6HPnj88wgLnRZvdV3cFfJd8doG%3DQ%40mail.gmail.com.

[James Schneider]

No problem. If you are happy with the functionality, then keep it simple and go with what you know.

However, I would highly, highly recommend that you do not rely on system level packages for application development (such as those installed as .deb packages). I would move your entire project into a Python virtual environment and manage all package versions via pip. It is definitely worth a Google...

If you have any plans to make it a long-standing project, future developers who look at your code will thank you.

-James

To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/trinity-c7c508ec-96c8-4c56-a002-196fe4f22bc8-1420578529489%403capp-gmx-bs48.