aldeano - 4 months ago 25

Python Question

I'm building an app to save data and some calculations made with that data, the idea is keep the data in one model and the calculations in other. So, the models are like this:

`class FreshData(models.Model):`

name = models.CharField(max_length=20)

one = models.IntegerField()

two = models.IntegerField()

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

Calculations()

Calculations.three = self.one + self.two

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

Calculations.save()

class Calculations(FreshData):

three = models.IntegerField()

I've got a

`ValueError`

Answer

You have just instantiated a class but not stored `calculations()`

in a variable, plus give your class names capital letters. Also your `super`

call is incorrect, it needs to reference your model and not `Blog`

.

The reason `self.one`

probably doesn't have a value is because you are doing the work before the call to the super save call. So instead do your work after saving "your self".

```
class FreshData(models.Model):
name = models.CharField(max_length=20)
one = models.IntegerField()
two = models.IntegerField()
def save(self, *args, **kwargs):
super(FreshData, self).save(*args, **kwargs)
calculations = Calculations()
calculations.three = self.one + self.two
calculations.save()
class Calculations(FreshData):
three = models.IntegerField()
```

Besides that I am not sure why you need to extend from `FreshData`

in your example `Calculations`

would inherit the `name`

, `one`

and `two`

fields from `FreshData`

. If you wanted to store the calculations only in a separate model then just store a reference back to the original data like so:

```
class Calculations(models.Model):
fresh_data = models.ForeignKey(FreshData)
three = models.IntegerField()
```

Then just make sure to pass in `fresh_data`

when instantiating your `Calculations`

:

```
def save(self, *args, **kwargs):
super(FreshData, self).save(*args, **kwargs)
calculations = Calculationrs(fresh_data=self)
...
```