小编Abd*_*oul的帖子

我有一个查询，如下所示，该查询返回特定课程（学年）中特定学期（学期）中特定班级的所有学生的成绩：

grades = Grade.objects.filter(term='First', student__in_class=1,session=1).order_by('-total')

然后是另一个对成绩进行注释的查询，以获取“总计”字段的总和。

grades_ordered = grades.values('student')\
    .annotate(total_mark=Sum('total')) \
    .order_by('-total_mark')

最初，一切正常，直到我从使用SQLite迁移到PostgreSQL时，以下错误开始出现。

错误：

function sum(character varying) does not exist
LINE 1: SELECT "sms_grade"."student_id", SUM("sms_grade"."total") AS...
                                         ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

编辑：这是我的模型

class Grade(models.Model):
    session = models.ForeignKey(Session, on_delete=models.CASCADE)
    term = models.CharField(choices=TERM, max_length=7)
    student = models.ForeignKey(Student, on_delete=models.CASCADE)
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE)
    fca = models.CharField(max_length=10)
    sca = models.CharField(max_length=10)
    exam = models.CharField(max_length=10)
    total = models.CharField(max_length=3, blank=True, null=True)
    grade = …