Django UNIQUE constraint fails with composite primary key

Question

I have searched for similar cases but they do not seem to question the model.save() method. I am new to Django and, following the standard methodology, have declared a class to map it to a ddbb (autoincremental id):

class Crudos(models.Model):

    cuenta = models.CharField(verbose_name='Cuenta', max_length=100)
    concepto = models.CharField(verbose_name='Concepto', max_length=100, default='indefinido')
    magnitud = models.FloatField(verbose_name='Magnitud')
    fecha = models.DateField(verbose_name='Fecha')
    documento = models.FilePathField(verbose_name='Documento')

    class Meta:
        unique_together = ('cuenta', 'fecha',)

The view that uses this class also populates the table:

for index, row in df.iterrows():
    data = Crudos(cuenta=row.Cuenta, concepto=row.Concepto, magnitud=row.Magnitud, fecha=row.Fecha, documento=row.Documento)
    data.save()

The dataframe is generated from a file and the process works without problems the first time. If I use the same file twice, then I get the error message of UNIQUE constrain failed. Models.save() should be an update instead of an insert when trying to insert a register with the same unique combination of values. It does so when the primary key is included in the data to save. The problem comes when there is a need for a composite primary key, as it is the case of the Crudo class, which is reflected by the unique_together condition. It seems Django is not able to interpret that condition as one to execute an update instead of an insert. As a way around that problem, the process works by:

for i, row in df.iterrows():
   register = Crudos.objects.get(cuenta=row.Cuenta, fecha=row.Fecha)
   if register:
      data = Crudos(id=register.id, cuenta=row.Cuenta, concepto=row.Concepto, magnitud=row.Magnitud, fecha=row.Fecha, documento=row.Documento)
      data.save()
   else:
      data = Crudos(cuenta=row.Cuenta, concepto=row.Concepto, magnitud=row.Magnitud, fecha=row.Fecha, documento=row.Documento)
      data.save()

This process forces a query before each save, slowing down the process significantly. Is there some other more efficient alternative? Is there a better way that allows Django to automatically execute as an update situations that violate the UNIQUE condition?

Thanks