Question

我正在为我的学校构建一个ratemyprofessors类型的应用程序，并且还要进行一些练习。

目前我的 models.py 如下所示：

from __future__ import unicode_literals

from django.db import models
from django.core.validators import MaxValueValidator, MinValueValidator
from django.contrib.auth.models import User
from django.utils import timezone

UNIVERSITIES = (
    .....
)

DEPARTMENTS = (
    .....
)

class Professor(models.Model):
    name = models.CharField(max_length=255)
    name_code = models.CharField(max_length=3, blank=True)
    university = models.CharField(max_length=3, choices=UNIVERSITIES)
    department = models.CharField(max_length=50, choices=DEPARTMENTS)
    total_rating_points = models.IntegerField(default = 0)
    number_of_reviews = models.IntegerField(default = 0)
    rating = models.FloatField(
        validators = [MinValueValidator(0.0), MaxValueValidator(5.0)],
        default = 0.0
    )

    def __str__(self):
        return self.name

SCORE_CHOICES = (
    .....
)

class Review(models.Model):
    author = models.ForeignKey(User, related_name='user_reviews')
    professor = models.ForeignKey(Professor, related_name='professor_reviews')
    created = models.DateTimeField(default=timezone.now)
    updated = models.DateTimeField(default = timezone.now)
    rating = models.IntegerField(default=1, choices=SCORE_CHOICES)
    text = models.TextField(blank=True)

    class Meta:
        unique_together = [('author', 'professor')]

    def __str__(self):
        return 'Professor: ' +self.professor.name +', Score: ' +str(self.rating)

    def save(self, *args, **kwargs):
        """
        Re-writing the save method to update the associated professor's
        rating as soon as a new Review object is created.
        Also accounts for review updates by the user.
        """
        if self.pk is None:
            # This means that this is a new object
            if self.professor:
                p = self.professor
                # Adjusting the total_rating_points and number of reviews
                p.total_rating_points += self.rating
                p.number_of_reviews += 1
                # Adjusting the rating
                p.rating = float(p.total_rating_points) / float(p.number_of_reviews)
                p.save()
        else:
            # This object already exists, so this is an update
            self.updated = timezone.now()
            **WHAT DO I DO NOW?**


        super(Review, self).save(*args, **kwargs)

您会看到，如果用户更新了他/她的评分，则必须相应调整教授的评分。由于这是应用程序的核心，我想在save（）方法中完成。如果它是一个非常新的评论，它很好地工作。但是，如何更新分数？

我的意思是我知道自己要做什么：

从教授的total_rating_point中减去以前的分数。
将新评级添加到total_rating_point
通过将其除以number_of_review计算评级。

但是，在更新期间，我究竟如何在save（）方法中检索以前的分数？还有一个更好，更有效率的做我想做的事情？谢谢！

Answer 1

要记住一些问题：如果用户删除了他们的帐户，评论等，该怎么办？以所示方式保持运行总计将是有问题的。

相反，我建议使用如下所示的结构;更新Review时，保存它，然后调用Professor的保存方法。 Professor次运行的新保存方法会计算到位的评论总数和计数，每次都会重新计算，因为它仍然只是针对几个查询与数据库联系。

from django.db.models import Sum

class Professor(models.Model):
    ...
    def save(self,*args,**kwargs):
        if self.pk: # prevent hitting the database unless professor already exists
            professor_reviews = Review.objects.filter(professor=self)
            # Adjusting the total_rating_points and number of reviews
            self.total_rating_points = professor_reviews.aggregate(Sum('rating'))
            self.number_of_reviews = professor_reviews.count()
            # Adjusting the rating
            self.rating = float(self.total_rating_points) / float(self.number_of_reviews)
        super(Professor,self).save(*args,**kwargs)

class Review(models.Model):
    ....         
    def save(self, *args, **kwargs):
        if self.pk: #already exists
            self.updated = timezone.now()
        super(Review, self).save(*args, **kwargs) 
        self.professor.save() # call after super writes the Review to the DB

Answer 2

使用post_save信号。

在教授的模型文件中：

from django.db.models.signals import post_save
from django.dispatch import receiver

@receiver(post_save,sender=Review)
def update_professor_by_review(*args,**kwargs):

 updated_review = kwargs['instance']
 reviewed_professor = updated_review.professor
  # ... update the reviewed_professor as needed according to the review instance
  reviewed_professor.save()
  return

如何为我的Django模型覆盖save（）方法以使其正确更新？

2 个答案: