Django how to compute the Percentage using annotate?

Question

I want to compute the total average per grading categories and multiply it by given number using annotate

this is my views.py

from django.db.models import F
gradepercategory = studentsEnrolledSubjectsGrade.objects.filter(grading_Period = period).filter(Subjects = subject).filter\
        (Grading_Categories__in = cate.values_list('id')).values('Grading_Categories').annotate( average_grade = Avg * F('Grading_Categories__PercentageWeight') / 100)

this is my models.py

class gradingCategories(models.Model):
    CategoryName = models.CharField(max_length=500, null=True)
    PercentageWeight = models.FloatField()


class studentsEnrolledSubjectsGrade(models.Model):
    GradeLevel = models.ForeignKey(EducationLevel, related_name='grade', on_delete=models.CASCADE,
                                   null=True, blank=True)
    Subjects = models.ForeignKey(Subject, related_name='subject', on_delete=models.CASCADE, null=True)
    Students_Enrollment_Records = models.ForeignKey(StudentsEnrolledSubject, related_name='student',
                                                    on_delete=models.CASCADE, null=True)
    Grading_Categories = models.ForeignKey(gradingCategories, related_name='category', on_delete=models.CASCADE,
                                           null=True, blank=True)
    grading_Period = models.ForeignKey(gradingPeriod, related_name='period', on_delete=models.CASCADE,
                                       null=True, blank=True)
    _dates  = models.CharField(max_length=255,null=True, blank=True)
    Grade = models.FloatField(null=True, blank=True)

For more info:

For example the average of Quiz(Grading categories) is 80 and it will multiply with 15(PercentageWeight)

Total = 80(Grading_Categories) * 15(PercentageWeight) total=total / 100

The result must be 12

Answer 1

You can do this using F Expresisons

From the Docs:

Reporter.objects.all().update(stories_filed=F('stories_filed') + 1)

So your example would be something like:

studentsEnrolledSubjectsGrade.objects.all().annotate(grade_percent=(.8 * F('Grading_Categories')) * (.15 * F('PercentageWeight') / 100)

(as a side note, it is django/python standards to use UpperCaseCamelCase for class names, and snake_case for field names. So StudentsEnrolledSubjectsGrade, and grading_categories) check out Pep-8 for the details)