Duplicate UUID if we enter a new input from html?

Question

getting error- i saved one form from html into my database while when i tried saving the next it gave me this error- IntegrityError at customer (1062, "Duplicate entry 'ad138e46-edc0-11va-b065-a41g7252ecb4'' for key 'customer.PRIMARY'") kindly explain my uuid in models.py is -

class customer(models.Model):
    customerid = models.CharField(default=str(uuid.uuid4()), max_length=500, primary_key=True)
    customername=models.CharField(max_length=100)

kindly help

Updated

Form.py

class createcustomerform(ModelForm):
    class Meta:
        model=customer
        fields=[
'customername']

Updated Models.py

import uuid
from uuid import UUID
from django.contrib.auth.models import User
from django.dispatch.dispatcher import receiver
from django.utils.translation import ugettext_lazy as _
from django.db.models.signals import pre_save
class customer(UUIDMixin,models.Model):
    customername=models.CharField(max_length=100)
    def __str__(self):
        return self.customername
class UUIDMixin(models.Model):
       uuid = models.UUIDField(blank=True,db_index=True,default=None,help_text=_('Unique identifier'),max_length=255,null=True,unique=True,verbose_name=_('UUID'))
       class Meta:
            abstract = True
@classmethod
def check_uuid_exists(cls, _uuid):
        #Determine whether UUID exists """
          manager = getattr(cls, '_default_manager')
          return manager.filter(uuid=_uuid).exists()
          
@classmethod
def get_available_uuid(cls):
        #Return an Available UUID """
          row_uuid = uuid.uuid4()
          while cls.check_uuid_exists(uuid=row_uuid):
            row_uuid = uuid.uuid4()
            return row_uuid       
@receiver(pre_save)
def uuid_mixin_pre_save(sender, instance, **kwargs):
    if issubclass(sender, UUIDMixin):
        if not instance.uuid:
            manager = getattr(instance.__class__, '_default_manager')
            use_uuid = uuid.uuid4()
        while manager.filter(uuid=use_uuid):
            use_uuid = uuid.uuid4()
            instance.uuid = use_uuid
            #Automatically populate the uuid field of UUIDMixin models if not already populated.

Answer 1

You should use a proper UUIDField and avoid setting a default value.

Make sure the value is set when the object is created, and also ensure that the value is unique - obviously the chance of duplication in a UUID is incredibly small which is the whole point.

You could create yourself a model mixin which will add a uuid to your model(s) and ensure that the value is set when the object is saved;

class UUIDMixin(models.Model):
    """
    Mixin for models contain a unique UUID, gets auto-populated on save
    """
    uuid = models.UUIDField(
        blank=True,
        db_index=True,
        # default=uuid.uuid4,
        # NB: default is set to None in migration to avoid duplications
        default=None,
        help_text=_('Unique identifier'),
        max_length=255,
        null=True,
        unique=True,
        verbose_name=_('UUID'),
    )

    class Meta:
        """Metadata for the UUIDMixin class"""
        abstract = True

    @classmethod
    def check_uuid_exists(cls, _uuid):
        """ Determine whether UUID exists """
        manager = getattr(cls, '_default_manager')
        return manager.filter(uuid=_uuid).exists()

    @classmethod
    def get_available_uuid(cls):
        """ Return an Available UUID """
        row_uuid = uuid.uuid4()
        while cls.check_uuid_exists(uuid=row_uuid):
            row_uuid = uuid.uuid4()
        return row_uuid


@receiver(pre_save)
def uuid_mixin_pre_save(sender, instance, **kwargs):
    """
    Automatically populate the uuid field of UUIDMixin models if not already
    populated.
    """
    if issubclass(sender, UUIDMixin):
        if not instance.uuid:
            manager = getattr(instance.__class__, '_default_manager')
            use_uuid = uuid.uuid4()
            while manager.filter(uuid=use_uuid):
                use_uuid = uuid.uuid4()
            instance.uuid = use_uuid

Based on your comment, let me explain more.

The above is an abstract model, meaning it doesn't create a table itself, it can just be used by other (concrete) models so that they can use what it defines.

It's a benefit of classes & inheritance. Allowing you to not duplicate code on things that will be useful on many models.

No, in the meta you'll see abstract = True. So you define your models like MyModel(UUIDMixin, models.Model) and it gets a uuid field from this abstract model along with whatever you define.

You would use it by doing something like this;

class Customer(UUIDMixin, models.Model):
    name = models.CharField(max_length=100)

If you really want to use the UUID as a primary key, maybe setup two mixins, a PrimaryUUIDMixin and the UUIDMixin, because on the whole a smaller value on the primary key may be more efficient.

You also mentioned Undefined variable: _

Typically in django you'd see an import at the top of the file like this;

from django.utils.translation import ugettext_lazy as _

This is then used to wrap strings so that they can be translated, for example, _("Hello")

Even if you don't translate your project, it's common to just include this anyway. You can read up on that here; https://docs.djangoproject.com/en/3.1/topics/i18n/translation/#standard-translation