AutoField & auto-incrementing

[Bruckner de Villiers]

Jason,

Thank you.  I have changed the title of this mail.  If I have read the 10 year old ticket 8576 correctly, it seems to be a DB restriction and I imagine that there are good reasons for this.  However, then I don’t understand the purpose of the AutoField given the restrictions and that the Django documentation is somewhat ambiguous/vague.

 

The options seem to be:

• Your uuid suggestion (which I have never worked with, but will explore)
• a function in the model that does the incrementing before committing – (anyone have some example code for this?).  The challenge would be to keep track of the last used ticket no.  I haven’t thought this through yet, but it does occur to me that one could use the id (maybe prefix the id with some starting digits) and write the record twice.  Probably barking up the wrong tree.

Regards,

 

Bruckner de Villiers

+27 83 625 1086

 

From: <django...@googlegroups.com> on behalf of Jason <jjohn...@gmail.com>
Reply to: <django...@googlegroups.com>
Date: Monday, 10 February 2020 at 14:55
To: Django users <django...@googlegroups.com>
Subject: Re: Adding a verbose_name to id field

 

Hmm.  TIL about https://code.djangoproject.com/ticket/8576

 

Would a a uuid for this, rather than an integer?  If not, you'll have to implement a save override to handle the incrementing yourself.

 

In addition, it might be worthwhile bringing this ticket up for discussion at https://groups.google.com/forum/#!forum/django-developers

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[Mike Dewhirst]

On 11/02/2020 7:06 pm, Bruckner de Villiers wrote:
>
> Jason,
>
> Thank you.  I have changed the title of this mail.  If I have read the
> 10 year old ticket 8576 correctly, it seems to be a DB restriction and
> I imagine that there are good reasons for this.  However, then I don’t
> understand the purpose of the AutoField given the restrictions and
> that the Django documentation is somewhat ambiguous/vague.
>
> The options seem to be:
>

> * Your uuid suggestion (which I have never worked with, but will
> explore)
> * a function in the model that does the incrementing before

> committing – (anyone have some example code for this?).  The
> challenge would be to keep track of the last used ticket no.  I
> haven’t thought this through yet, but it does occur to me that one
> could use the id (maybe prefix the id with some starting digits)
> and write the record twice. Probably barking up the wrong tree.
>

I have a need for a unique identifier in one of my models. I save the id
to a separate field in the same model. In my case I don't need to see
the value. It just needs to be unique and never change. It is used for
generating the equivalent of a bar-code in a different model.

    def save(self, *args, **kwargs):
        if self.id:
            if not self.formulation_no:
                self.formulation_no = str(self.id)
        super().save(*args, **kwargs)

A downside to this is the formulation number stays blank until the next
save after creating the record. The upside is that the id isn't going to
change so uniqueness isn't lost. Another way of doing this is to copy
the id when it is first required and formulation_no is still blank.

I have also been known to use a weird mechanism for user-resequencing
records in a display. In that case they are lesson sequences within a
course and question sequences within a lesson. I use floats and strings
in a pair of fields so if that is of interest just ask.

Cheers

Mike

> *

>
> Regards,
>
> Bruckner de Villiers
>
> +27 83 625 1086
>

> *From: *<django...@googlegroups.com> on behalf of Jason
> <jjohn...@gmail.com>
> *Reply to: *<django...@googlegroups.com>
> *Date: *Monday, 10 February 2020 at 14:55
> *To: *Django users <django...@googlegroups.com>
> *Subject: *Re: Adding a verbose_name to id field

>
> Hmm.  TIL about https://code.djangoproject.com/ticket/8576
>
> Would a a uuid for this, rather than an integer?  If not, you'll have
> to implement a save override to handle the incrementing yourself.
>
> In addition, it might be worthwhile bringing this ticket up for
> discussion at https://groups.google.com/forum/#!forum/django-developers
>
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[Bruckner de Villiers]

Jason,

I cracked the uuid option – what an ugly ticket no.!!! - 0273051c-af54-4baf-91b9-651701d9020c

 

Bruckner de Villiers

083 625 1086

[Bruckner de Villiers]

Mike,
Clever. The save function seems to write a string of the id. So, self.formulation_no = "50-" + str(self.id) could also work?

Bruckner de Villiers
+27 83 625 1086

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[Mike Dewhirst]

On 12/02/2020 6:11 pm, Bruckner de Villiers wrote:
> Mike,
> Clever. The save function seems to write a string of the id. So, self.formulation_no = "50-" + str(self.id) could also work?

Fact is you can do anything you like. Instead of what I showed you could
call any method you care to write either before or after the super()
call in model.save()

You can also use pre_save and post_save signals to do much the same thing.

Where I need to do such things my preference is a model save method
which looks like this ...

def save(self, *args, **kwargs):
    self._pre_save()    # stuff to be computed pre-save
    super().save(*args, **kwargs)
    self._post_save()   # stuff to be done after the save

Specifically, yes my requirement was to have a models.CharField so
str(self.id) was the way to satisfy that.

Cheers

Mike

[Bruckner de Villiers]

Thanks Mike - I have just become cleverer.

Bruckner de Villiers
083 625 1086

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[J. Guadalupe Chavez]

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[Mike Dewhirst]

On 12/02/2020 6:54 pm, Mike Dewhirst wrote:
> def save(self, *args, **kwargs):
>     self._pre_save()    # stuff to be computed pre-save
>     super().save(*args, **kwargs)
>     self._post_save()   # stuff to be done after the save

Also of occasional value ...

def save(self, *args, **kwargs):
    whatever = self._pre_save()    # returns something needed by
_post_save()
    super().save(*args, **kwargs)
    self._post_save(whatever)      # gets input across the super ()
boundary

Cheers

Mike

[Bruckner de Villiers]

Useful, thanks Mike.

Bruckner de Villiers
083 625 1086

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