Django, how to update DB based on the foreignkey value?

Question

I have tried in Models (with "def save") but give me error "int() argument must be a string, a bytes-like object or a number, not 'Length'".

I have fields "length" and "width" in foreignkey. And I want when submit a form to add value of "squaremeter" field to DB calculated based on the length and width selected

def save(self, *args, **kwargs):        
        self.squaremeter = self.length * self.width
        self.squaremeter.save()        
        super(Product, self).save(*args, **kwargs)

For two weeks I'm stuck in this issue

Update:

class Product(models.Model):
    length      = models.ForeignKey(Length, on_delete=models.SET_NULL, null=True, blank=True)
    width       = models.ForeignKey(Width, on_delete=models.SET_NULL, null=True, blank=True)
    squaremeter = models.DecimalField(max_digits=6, decimal_places=4, blank=True)

Update2:

class Length(models.Model):
    value    = models.PositiveSmallIntegerField(unique=True)

class Width(models.Model):
    value    = models.PositiveSmallIntegerField(unique=True)    

Answer 1

Edit your save method like this,

def save(self, *args, **kwargs):        
    self.squaremeter = self.length.value * self.width.value       
    super(Product, self).save(*args, **kwargs)