Django - what is best practice - Calculating field values [closed]

Question

I have a model in Django which has three fields which are calculated based on the value of one field. The value of one of the fields in particular will require records from another table to be queried ( I will be using the average value of the last ten values).

I am unsure as to where is the best place to put this functionality, in the model class, in a model form, in a view?

Any advice would be appreciated - thanks

The model looks like this:

class slide_library(models.Model):

    slide_name = models.Charfield(max_length = 6, primary_key = True)
    reference_value = models.FloatField(default= '0')
    last_mean = models.FloatField(default= '0')
    esd = models.FloatField(default= '0')
    criteria = models.Charfield(max_length= 10)

Answer 1

You can avoid using signals(you should use signals as last resource), overriding model save method and calculating your values before store the instance.

class slide_library(models.Model):

    slide_name = models.Charfield(max_length = 6, primary_key = True)
    reference_value = models.FloatField(default= '0')
    last_mean = models.FloatField(default= '0')
    esd = models.FloatField(default= '0')
    criteria = models.Charfield(max_length= 10)

    #Overriding
    def save(self, *args, **kwargs):
        #set here your calculated attributes
        self.my_stuff = 'something I want to save in that field'
        super(slide_library, self).save(*args, **kwargs)

Also, if they are calculated attributes, think if you really need to store it in DB, you can calculate on the fly.

You can use decorator @cached_property, as django docs says

The @cached_property decorator caches the result of a method with a single self argument as a property. The cached result will persist as long as the instance does, so if the instance is passed around and the function subsequently invoked, the cached result will be returned.

Consider a typical case, where a view might need to call a model’s method to perform some computation, before placing the model instance into the context

from django.utils.functional import cached_property

class slide_library(models.Model):

    slide_name = models.Charfield(max_length = 6, primary_key = True)
    reference_value = models.FloatField(default= '0')
    last_mean = models.FloatField(default= '0')
    esd = models.FloatField(default= '0')
    criteria = models.Charfield(max_length= 10)

    @cached_property
    def derivate_field_1(self):
        #Here goes all ur stuff to calculated your field
        value = ....your calculated value
        return value