使用 ForeignKey 对链接模型进行排序答案

【问题标题】：Sort through linked model with ForeignKey使用 ForeignKey 对链接模型进行排序
【发布时间】：2019-07-09 10:42:02
【问题描述】：

我需要通过CreditPayment的关联模型对Credit记录进行排序。

models.py

class Credit(models.Model):
    hot = models.BooleanField(default=False)


class CreditPayment(models.Model):
    credit = models.ForeignKey(Credit)
    period_to = models.PositiveIntegerField()
    rate = models.DecimalField(max_digits=7, decimal_places=2)

views.py

credits = credits.filter(hot=False).distinct().order_by(...)

输入数据示例：

Credit #1:
CreditPayment #1:
period_to = 12
rate = 10

CreditPayment #2:      (minimal)
period_to = 10
rate = 8

CreditPayment #3:      
period_to = 9
rate = 10

Credit #2:
CreditPayment #1:        (minimal)
period_to = 6
rate = 20

CreditPayment #2:
period_to = 9
rate = 20

Credit #3:
CreditPayment #1:
period_to = 12
rate = 8

CreditPayment #2:
period_to = 9
rate = 11

CreditPayment #3:       (minimal)
period_to = 9
rate = 8

因此，样本减少到：

Credit #1:    
CreditPayment #2:
period_to = 10
rate = 8

Credit #2:
CreditPayment #1:
period_to = 6
rate = 20

Credit #3:
CreditPayment #3:
period_to = 9
rate = 8

结果：

学分 #3 -> 学分 #1 -> 学分 #2

如您所见，最初为每个Credit 选择了最小CreditPayment（credits at views.py）。然后，根据这些最小CreditPayment，对所有Credit进行排序。如果两个条目的比率相等，则通过period to 比较这些条目。据我了解，在这里您需要以某种方式应用聚合。

【问题讨论】：

标签： django django-models django-views

【解决方案1】：

使用annotate() & prefetch_related()

models.py

class CreditPayment(models.Model):
    credit = models.ForeignKey(Credit, related_name='creditpayments')
    ....

views.py

from django.db.models import Max, Min, Prefetch

prefetch = Prefetch('creditpayments', CreditPayment.objects.all())
sorted_credits = Credit.objects.all().prefetch_related(prefetch).annotate(min_rate=Min('creditpayments__rate')).annotate(max_period_to=Max('creditpayments__period_to')).order_by('min_rate', '-max_period_to')

【讨论】：