Django model method with self instance

Question

I am trying to move some functional logic into a model's method (instead of in the views), where I think it belongs:

class Spans(models.Model):
    snow = models.IntegerField()
    wind = models.IntegerField()
    exposure = models.CharField(max_length=1)
    module_length = models.IntegerField()
    max_roof_angle = models.IntegerField()
    building_height = models.IntegerField()
    importance_factor = models.IntegerField()
    seismic = models.DecimalField(max_digits=4, decimal_places=2)
    zone = models.IntegerField()
    profile = models.CharField(max_length=55)
    tilt_angle = models.IntegerField()
    span = models.DecimalField(max_digits=18, decimal_places=15)

    def get_spans(self, snow_load, wind_speed, module_length):
        """
        Function to get spans.
        """
        spans = self.objects.filter(
            snow=snow_load,
            wind=wind_speed,
            exposure='B',
            module_length__gte=module_length,
            max_roof_angle=45,
            building_height=30,
            importance_factor=1,
            seismic=1.2,
            zone=1,
            profile='worst'
        ).order_by('snow', 'module_length', 'span')
        return spans

However, I am a little unsure how to call it. In the shell I have tried:

>>> possible_spans = Spans.get_spans(0, 85, 54)

or

>>> possible_spans = Spans.get_spans(Spans, 0, 85, 54)

but I get an error:

TypeError: unbound method get_spans() must be called with Spans instance as first argument (got int instance instead) 

or

TypeError: unbound method get_spans() must be called with Spans instance as first argument (got ModelBase instance instead)  

I know I am missing something fundamental in python logic here, but I'm not sure what it is.

Any help would be much appreciated.

Answer 1

Method needs to be called either with the instance, as the method call makes it as the default self arg. For example,

instance = Spans.objects.get(id=1)
instance.get_spans(0, 85, 54)

Or if you can want to call using class itself. You would have to pass the instance manually

instance = Spans.objects.get(id=1)
Spans.get_spans(instance, 0, 85, 54)

Update: What you are looking for could be achieved using a static method

@staticmethod
def get_spans(snow_load, wind_speed, module_length):
        """
        Function to get spans.
        """
        spans = Spans.objects.filter(
            snow=snow_load,
            wind=wind_speed,
            exposure='B',
            module_length__gte=module_length,
            max_roof_angle=45,
            building_height=30,
            importance_factor=1,
            seismic=1.2,
            zone=1,
            profile='worst'
        ).order_by('snow', 'module_length', 'span')
        return spans

Now you could directly do,

Spans.get_spans(0, 85, 54)