use of property decorator in Django to calculate Sum

Question

I have two moldes in which one model consists of file field. I have to calculate total size of the files in the parent model within the property decorator. I think this could also be done inside the serializer class using the serializer method but I want within the property decorator.

class Package(models.Model):   

    name = models.CharField(max_length=50,blank=True)
    />....................other fields.........../

    **/.........problem is here..../**
    @property
    def folder_size(self):
        all = self.details.all()
        
        all.annotate()
        return total

Here I need help to calculate the sum of all files sizes of a single package.

Class Details(models.Model):
    package = models.ForeignKey(Package, on_delete=models.CASCADE,null=True,
                               related_name='details')
    notes = models.FileField(blank=True, null=True)

    @property
    def file_size(self):
        return self.file.size