limit one record to be default in django model - django

Question

What is the best way to limit only one record to be default in django
I have a model where i have a flag for default
class BOMVersion(models.Model):
name = models.CharField(max_length=200,null=True, blank=True)
material = models.ForeignKey(Material)
is_default = models.BooleanField(default=False)
I want to have only one value to be default for the same material but this material can have a lot of non default ones.

Answer 1

It was odd that this question was not addressed more often. If I have a default record, I want to record that in the class as a member variable. And to determine if an instance is the default, I want to compare the class default member variable with the instance id. Unfortunately I could not figure out how to access class variables and instance variables nicely in the same class function in Python (may be someone can comment), but this does not require to hit the database for a default or store a bunch of records pointing to a default. Just one member variable in the class.
After I wrote this, I realized every time the application is restarted, default is reset to None, so you will have to store this in a database. I have updated my answer accordingly. However, checking that the member variable is not null, and only hitting the database if it is would reduce hits here. The model I used was:
class RecordOfInterest(models.Model):
"""
Record Records of interest here. Stores ID, and identifying character
"""
# asume maximum 64 character limit for the model.
model_name = models.CharField(max_length=64, unique=True)
record_no = models.IntegerField()
human_ident = models.CharField(max_length=64, help_text='How is this of interest')
# Id it as default, deposit, ... Don't bother indexing, as there will only be a few
def __unicode__(self):
return u'Model %s record %d for %s' % (self.model_name, self.record_no, self.human_ident)
class Meta:
unique_together = ('model_name', 'human_ident')
class Product(models.Model):
"""
Allow one product record to be the default using "Product.default = prod_instance"
check default with "Product.is_default(prod_instance)"
"""
default = None # set this to id of the default record
cart_heading = models.CharField(max_length=64, blank=True)
country = CountryField()
pricing = models.ForeignKey(
'Price', blank=True, null=True, related_name='visas', on_delete=models.SET_NULL)
#classmethod
def is_default(cls, obj):
if cls.default_no == None:
try:
cls.default_no = RecordOfInterest.objects.get(model_name=cls.__name__, human_ident='default')
except RecordOfInterest.DoesNotExist:
default_no = None
return cls.default_no == obj.id
#classmethod
def set_default(cls, obj):
try:
default_rec = RecordOfInterest.objects.get(model_name=cls.__name__, human_ident='default')
except RecordOfInterest.DoesNotExist:
RecordOfInterest.objects.create(model_name=cls.__name__, record_no=obj.id, human_ident='default')
else:
if default_rec.record_no != obj.id:
default_rec.record_no = obj.id
default_rec.save()
cls.default_no = obj.id
return

Answer 2

Saving the ID in settings.py if it is static.
Save it into a separate "default" table with one record (or use the most recent) if it's dynamic.
Save the default in another table like this:
class BOMVersion(models.Model):
name = models.CharField(max_length=200,null=True, blank=True)
material = models.ForeignKey(Material)
class BOMVersionDefault(model.Models)
time_set= models.Datetime(auto_created=True)
default_material = models.ForiegnKey(Material)
To query:
default = BOMVerDefault.objects.latest(time_set).get().default_material
If you have several material types that each need a default then default_material would be a field in a material-type table.

Answer 3

Getting one record to be default in a table is most basic requirement we developer come face to face, after spending couple of hours over it, i think a neat and clean solution in django would be update all records to default false if current form instance has default value to be "true" and then save the record.
class FeeLevelRate(TimeStampedModel):
"""
Stores the all the fee rates depend on feelevel
"""
feelevel = models.ForeignKey(FeeLevel, on_delete= models.PROTECT)
firstconsultfee = models.DecimalField(_('First Consultation Charges'),max_digits=10,decimal_places=2,blank=True)
medcharges = models.DecimalField(_('Medicines Charges per Day'),max_digits=10,decimal_places=2,blank=True)
startdate = models.DateField(_("Start Date "), default=datetime.date.today)
default_level = models.BooleanField(_('Is Default Level?'),default=False)
class Meta:
constraints = [
models.UniqueConstraint(fields=["feelevel","startdate"], name='unique_level_per_date'),
]
def __str__(self):
return "%s applicable from (%s)" % ( self.feelevel, self.startdate.strftime("%d/%m/%Y"))
class FeeLevelRateCreate(CreateView):
model = FeeLevelRate
fields = ['feelevel', 'firstconsultfee', 'medcharges', 'startdate', 'default_level']
context_object_name = 'categories'
success_url = reverse_lazy('patadd:feerate_list')
def form_valid(self, form):
# Update all the default_level with false.
#UserAddress.objects.filter(sendcard=True).update(sendcard=False)
if form.instance.default_level:
FeeLevelRate.objects.filter(default_level=True).update(default_level=False)
return super().form_valid(form)