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 On Jan 6, 2015 3:30 AM, <sneak...@gmx.net> wrote: > 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 > > -- > You received this message because you are subscribed to the Google Groups > "Django users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to django-users+unsubscr...@googlegroups.com. > To post to this group, send email to django-users@googlegroups.com. > Visit this group at http://groups.google.com/group/django-users. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-users/trinity-29b3450f-f209-4d1d-8406-9f7ff205766a-1420541277592%403capp-gmx-bs67 > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscr...@googlegroups.com. To post to this group, send email to django-users@googlegroups.com. Visit this group at http://groups.google.com/group/django-users. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/CA%2Be%2BciWtP93C21m57dSi8c6HPnj88wgLnRZvdV3cFfJd8doG%3DQ%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
