Django如何使用注释计算百分比？

Question

我想计算每个评分类别的总平均值，并使用 annotate 将其乘以给定的数字

这是我的 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)

这是我的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)

欲了解更多信息：

例如测验（评分类别）的平均值是 80，它将乘以 15（百分比权重）

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

结果必须是 12

Answer 1

您可以使用F 表达式执行此操作

从文档：

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

所以你的例子是这样的：

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

（作为旁注，使用 UpperCaseCamelCase 作为类名，并使用 snake_case 作为字段名是 django/python 标准。因此 StudentsEnrolledSubjectsGrade 和 grading_categories）查看Pep-8了解详细信息）