Save, Signals and Models

[Victor Loureiro Lima]

Here is the deal:

class MyModel ( models.Model ):

  title = models.CharField( max_length = 100 )

  only_me = models.BooleanField( default = False )

 Question: Whats the proper way to guarantee that no matter how many MyModel's are available in the database, only one of them

will have the only_me set as True? To further clarify things: In the admin, whenever I check the only_me checkbox, and save my model, all other models of this class will have to have its own only_me field set to false.

 As far as I know, there is no other way of doing this unless I iterate over all MyModel' s objects and uncheck them if they are checked, save them, then afterwards check the model that I am actually saving setting the only_me field to True.

 I tried doing this on the actual save() of the model, no success. Everytime I called save on iterated objects, I, of course, got the maximum recursive depth error thrown at me. 

 Fair enough, I quickly thought about signals, hooking my function to post_save(), however I inevitabilly stumbled upon the same

problem: When I called save() on the iterated objects the post_save signal got sent, I would step again in the same function, thus

no cookie for me.

 I jumped over to overriding AdminForm' s save() method, so that I would iterate there on the models unchecking them if necessary, and them returning the proper object, but I stopped that and I said to myself that I must be doing something really stupid, so Im coming to you guys: What would the propper way of doing this?

Thanks in Advance,

Victor Lima

[Gabriel Reis]

Hey Victor,

I can think that a trivial way (I am not sure if it is the best) to do that is to overwrite the save() method of your model:

class MyModel(models.Model):

    title = models.CharField(max_length=100)

    only_me = models.Boolean(default=False)

    def save(self):

        if self.only_me:

            only_me_true = MyModel.objects.filter(only_me=True)

            for obj in only_me_true:

                obj.only_me = False

                obj.save()

        MyModel.save(self)

I would go for that solution. It is easy to implement, it seems clean and it is easy to test as well.

Cheers dude!

Gabriel de Carvalho Nogueira Reis
Software Developer
+44 7907 823942

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[Victor Loureiro Lima]

 I think that will not work,when I call the obj.save() I will inevitably step again into my own save() method and the only_me wont be set as False yet, thus the same problem would occur again and again until I get the maximum depth error. I dont think thats an option, ahev you tested this solution? I am far from my box right now, so I cant test it.

Victor Lima

2010/1/15 Gabriel Reis <gabri...@gmail.com>

[Mike Ramirez]

Add in an if statement, one that while cycling through the only_me=True list,
ignores the current model you're saving.

def save(self):
if self.only_me:
only_me_true = MyModel.objects.filter(only_me=True)
for obj in only_me_true:

if obj.id != self.id:

obj.only_me = False
obj.save()
MyModel.save(self)

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[Victor Loureiro Lima]

I did exactelly that, instead of using id I used the slug, which is also unique, so I skipped it,

but the problem still occurs. Either my logic is wrong, or some dumb mistake, but I think that wouldnt work either.

Victor Lima

2010/1/15 Mike Ramirez <gufy...@gmail.com>

[Mike Ramirez]

On Friday 15 January 2010 17:08:06 Mike Ramirez wrote:
> Add in an if statement, one that while cycling through the only_me=True
> list, ignores the current model you're saving.
>
> def save(self):
> if self.only_me:
> only_me_true = MyModel.objects.filter(only_me=True)
> for obj in only_me_true:
> if obj.id != self.id:
> obj.only_me = False
> obj.save()
> MyModel.save(self)
>

Sorry for the bad formatting -- hope this helps.

def save(self, *args, **kwargs):

if self.only_me:
only_me_true = MyModel.objects.filter(only_me=True)
for obj in only_me_true:
if obj.id != self.id:
obj.only_me = False
obj.save()

super(MyModel, self).save(*args, **kwargs)

Mike
--
What upsets me is not that you lied to me, but that from now on I can no
longer believe you.
-- Nietzsche

[Mike Ramirez]

On Friday 15 January 2010 17:11:23 Victor Loureiro Lima wrote:
> I did exactelly that, instead of using id I used the slug, which is also
> unique, so I skipped it,
> but the problem still occurs. Either my logic is wrong, or some dumb
> mistake, but I think that wouldnt work either.
>
> Victor Lima
>

I just rechecked the django docs, the reason post_save() doesn't work is cause
post save is at the _end_ of the save method, end isn't after the save is
executed, but after all the instructions are executed in the save method. Yes
everytime you call save(), even in the loop that bit of code is getting called
all the time.

You should update this in the model admin after the save is complete, send a
custom single or call a manager method (mangers are meant ot work at the table
level, as opposed to models working on the row level) or put this code bit in
a function you call in the model admin after saving. Ideally I would put it
in a custom singal and send that after the save, where ever I needed it.

Mike
--
Do people know you have freckles everywhere?

[Eric Chamberlain]

def save(self, force_insert=False, force_update=False):
if self.only_me is True:
# if this is the new default, set others to False
MyModel.objects.exclude(pk=self.pk).update(only_me=False)

super(MyModel, self).save(force_insert, force_update) # Call the "real" save() method.

--
Eric Chamberlain, Founder
RF.com - http://RF.com/

[Victor Loureiro Lima]

Thanks Eric, that worked like a charm. I've never got to use the update, thus I had no idea of its existence, fair enough that is exactelly what I needed.

Thank you very much, and other who have answered this thread.
Victor Lima

2010/1/16 Eric Chamberlain <er...@rf.com>

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