Django signals post_save update

问题

When the student enrollment record (discount type) is updated the student discount (discount type) is also updated, but what should i do if i want to update the student enrollment record (discount type) using student discount (discount type)

This is my models.py

class studentDiscount(models.Model):
    Students_Enrollment_Records = models.ForeignKey(StudentsEnrollmentRecord, related_name='+',
                                                    on_delete=models.CASCADE, null=True)
    Discount_Type = models.ForeignKey(Discount, related_name='+', on_delete=models.CASCADE, null=True,blank=True)

    @receiver(pre_save, sender=StudentsEnrollmentRecord)
    def get_older_instance(sender, instance, *args, **kwargs):
        try:
            instance._old_instance = StudentsEnrollmentRecord.objects.get(pk=instance.pk)
        except StudentsEnrollmentRecord.DoesNotExist:
            instance._old_instance = None

    @receiver(post_save, sender=StudentsEnrollmentRecord)
    def create(sender, instance, created, *args, **kwargs):
        if not created:
            older_instance = instance._old_instance
            if older_instance.Discount_Type != instance.Discount_Type:

                studentDiscount.objects.filter(
                    Students_Enrollment_Records=instance
                ).delete()
            else:
                return None

        discount = studentDiscount.objects.filter(Discount_Type=instance.Discount_Type)
        if created:
            print("nagsulod")
            studentDiscount.objects.create(
                Students_Enrollment_Records=instance,
                Discount_Type=instance.Discount_Type)
        else:
            studentDiscount.objects.create(
                Students_Enrollment_Records=instance,
                Discount_Type=instance.Discount_Type)

    def __str__(self):
        suser = '{0.Students_Enrollment_Records}'
        return suser.format(self)

class StudentsEnrollmentRecord(models.Model):
    Student_Users = models.ForeignKey(StudentProfile, on_delete=models.CASCADE,null=True)
    School_Year = models.ForeignKey(SchoolYear, related_name='+', on_delete=models.CASCADE, null=True, blank=True)
    Courses = models.ForeignKey(Course, related_name='+', on_delete=models.CASCADE, null=True, blank=True)
    strands = models.ForeignKey(strand, related_name='+', on_delete=models.CASCADE, null=True, blank=True)
    Section = models.ForeignKey(Section, related_name='+', on_delete=models.CASCADE, null=True,blank=True)
    Payment_Type = models.ForeignKey(PaymentType, related_name='+', on_delete=models.CASCADE, null=True)
    Discount_Type = models.ForeignKey(Discount, related_name='+', on_delete=models.CASCADE, null=True,blank=True)
    Education_Levels = models.ForeignKey(EducationLevel, related_name='+', on_delete=models.CASCADE,blank=True,null=True)
    ESC = models.ForeignKey(esc, on_delete=models.CASCADE,null=True,blank=True)
    Remarks = models.TextField(max_length=500,null=True,blank=True)

class Discount(models.Model):
    Code=models.CharField(max_length=500,blank=True)
    Discount_Type=models.CharField(max_length=500,blank=True)
    Remarks=models.TextField(max_length=500,blank=True)
    TuitionFee_Discount_Amount=models.FloatField(null=True,blank=True)
    TuitionFee_Discount_Rate = models.FloatField(null=True,blank=True)
    Miscellaneous_Discount_Amount=models.FloatField(null=True,blank=True)
    Miscellaneous_Discount_Rate = models.FloatField(null=True,blank=True)

if i update the student enrollment record (Discount_Type)

the student discount (discount type) update as well

but when i update the student discount (discount_type)

the student enrollment record (discount type) doesn't update

回答1:

You need to write a signal handler for when the studentDiscount instance is created or updated. You'd need similar methods to studentDiscount.create, but it would need to exist on StudentsEnrollmentRecord and the sender be studentDiscount.

来源：https://stackoverflow.com/questions/59899514/django-signals-post-save-update