[FIXED] Django: I want to create a self-generated code based on previous records and a sequential number

Issue

I'm a coding noob and I'm writing a website in Django for Project and task management, and I want to generate a project code field in a 'Project' model automatically based on previous records.

The project code field should look like this: DDD-YY-SS, where DDD is department code, YY is 2-digit year number and SS the sequence number.

I'm trying to include the code generator in a custom save method like this:

class ProjectModel(models.Model):

    name = models.CharField(_("project name"), max_length=50)
    sponsor = models.CharField(_("sponsor"), max_length=50)
    manager = models.ForeignKey(StaffModel, verbose_name=_("project manager"), null=True, on_delete=models.SET_NULL, related_name='project_manager')
    lead = models.ForeignKey(StaffModel, verbose_name=_("technical lead"), null=True, on_delete=models.SET_NULL, related_name='tech_lead')
    projectdate = models.DateField(_("start date"), auto_now=False, auto_now_add=False)
    department = models.ForeignKey(DepartmentModel, verbose_name=_("department"), null=True, on_delete=models.SET_NULL)
    sites = models.ManyToManyField(OfficeModel, verbose_name=_("sites"))
    code = models.CharField(_("project code"), max_length=50, unique=True)
    partner = models.ForeignKey(CompanyModel, verbose_name=_("partner"), null=True, on_delete=models.SET_NULL)
    status = models.ForeignKey(ProjectStatusModel, verbose_name=_("status"), null=True, on_delete=models.SET_NULL)

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

        dpt = str(self.department.code)
        yy = str(self.projectdate.year)[-2:]
        filter_kw = '{}-{}-'.format(dpt, yy)
        lastrec = ProjectModel.objects.filter(code__startswith=filter_kw).last()

        if lastrec == None:
            lastrec = '00'
        else:
            lastrec = str(lastrec.code)[-2:]
 
        newnum = "{:02d}".format(int(lastrec)+1)
        self.code = '{}{}'.format(filter_kw, str(newnum))

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

But I think this code is... sketchy? I feel like using too many auxiliaries or it just won't work. Is there a better way of doing this? Should I include the generating function in the model or should I move this to the View or tag functions in the templates?

Thanks a lot. My head is a noodle now.

Solution

Finally I found a way of doing this by checking previous records and also preventing the "Project Code" (or "serial number") to update itself when modifying record data.

def save(self, *args, **kwargs):
    dpt = str(self.department.code) #'department' is ForeignKey, has a 'code' field consisting of a 2-3 letter code.
    yy = str(self.projectdate.year)[-2:] #gets the last 2 digits from the project start date year.
    filter_kw = '{}-{}-'.format(dpt, yy) #makes a database query to find any matching Dept-Year combination in the 'project code' field
    lastrec = ProjectModel.objects.filter(code__startswith=filter_kw).last() #get the last record that has the same combination.
    
    if not self.pk: #this keeps the following instructions from executing if there already is a project code, i.e.: when updating record information.

        if lastrec == None:
            lastrec = '00'
            newrec = int(lastrec)+1
        elif lastrec.code == self.code:
            lastrec = str(lastrec.code)[-2:]
            newrec = int(lastrec)
        else:
            lastrec = str(lastrec.code)[-2:]
            newrec = int(lastrec)+1
        
        newnum = "{:02d}".format(int(newrec)) #the record should be in Charfield format, 2-digits, non-unique just in case there is a number above 99 (very unlikely)
        self.code = '{}{}'.format(filter_kw, str(newnum))
    
    elif self.code == None:
        lastrec = '00'
        newrec = int(lastrec)+1
    
        newnum = "{:02d}".format(int(newrec))
        self.code = '{}{}'.format(filter_kw, str(newnum))
    else:
        pass

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

Answered By - Victor Donoso