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I'm posting to a backend Django REST API using a webapp developed in Angular 7 as front end. Basically I have a supertype model Factura for two subtypes, FacturaA and FacturaE, they are both related by a OneToOneField.

The models in question look like this:

class Factura(models.Model):

    fecha = models.DateField()

    contrato = models.ForeignKey(Contrato, related_name='facturas', on_delete=models.CASCADE)

    pagado = models.BooleanField(default=False)



def __str__(self):

    return 'Factura %s: %s - %s' % (self.id, self.fecha, self.contrato)



class FacturaE(models.Model):

    factura = models.OneToOneField(

        Factura,

        on_delete=models.CASCADE,

        primary_key=True,

    )

    kwh = models.DecimalField(max_digits=100, decimal_places=2)

    monto = models.DecimalField(max_digits=100, decimal_places=2, null=True, blank=True)



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

        #calcula el monto

        if self._state.adding is True:

            self.monto = float(self.kwh) * 4.0588

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

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



    class Meta:

        verbose_name = 'Factura Electrica'

        verbose_name_plural = 'Facturas Electricas'



    def __str__(self):

        return 'Electricidad: %s - %s' % (self.factura, self.monto)

In my frontend app, I POST by first creating a supertype object Factura, and then using the added key to post a FacturaE like so:

  addFactura(fecha: Date, kwh: number, monto: number) {

    this.factura = new Factura(0, fecha, false, this.id);

    this.pagoService.addFactura(this.factura).subscribe(f => {

      this.facturaElectrica = new FacturaEAdd(f.id, kwh, monto);

      this.pagoService.addFacturaElectrica(this.facturaElectrica).subscribe();

    });

  }

Adding a Factura followed by a FacturaE manually in the Django Admin produces no error, however doing this manually through the API or through my frontend app gives me the following error.

django.db.utils.IntegrityError: UNIQUE constraint failed: CasadelaAPI_facturae.factura_id.

Despite this error, and despite returning a 500 HTTP Status Code, the object is still created regardless of the method I use. I would like to know what's causing the error and how to fix it.

Edit:

Solved, super being called twice in save() method overwrite, backend was trying to save the FacturaE object twice, the second super().save(*args, **kwargs) was producing the error

The error is because you are violating OneOnOne attribute by creating a FactoraE with an existing Factora object assigned to it.

So the code is throwing Unique constraint failed ....

Please check below steps:

• be sure that you are not trying to create another FactoraE with a Factora object that already another FactoraE is existed with same Factora Item.




• maybe in your UI code, in service (which is not provided here) you are not passing Factora.id correctly, so it is the same as previous requests, causing this error.

Update
Calling super two times is not recommended and it may causing the problem.