如何在 Django 中使用左外连接？

Question

我有两个模型

class Business(models.Model):
    BID = models.IntegerField(primary_key=True, default=datetime.now().strftime("%d%y%H%S%m%M%f"))
    BusinessName = models.CharField(max_length=150)
    ContactPerson = models.CharField(max_length=50)

class BusinessComment(models.Model):
    BID = models.ForeignKey(Business, blank=True, null=True)
    Rating = models.IntegerField(blank=True, null=True)
    Comment = models.TextField(blank=True, null=True)

class BusinessHours(models.Model):
    BID = models.ForeignKey(Business, blank=True, null=True)
    Day = models.IntegerField(blank=True, null=True)
    StartHour = models.CharField(max_length=8, blank=True, null=True)
    EndHour = models.CharField(max_length=8, blank=True, null=True)

我想在单个对象中访问低于两个值的值 models.this 概念是左外连接，但我可以在具有以下所有值的单个对象中实现吗？

BID,BusinessName,ContactPerson,AVG(Rating),Count(Number of Comment) (Condition is Business.BID = BusinessComment.BID)

业务Table数据

BID BusinessName    ContactPerson
1   First            First
2   Second           Second

营业时间Table数据

id    BID   Rating  Comment
1      1    3      Comment1
2      1    5      Comment2
3      2    4      Comment3
4      2    5      Comment4

那么结果应该在 Object As:

BID Businame ContactPerson Rating Comment
1    First      First        4      2
2    Second      Second      4.5     2

我已经试了很多次了，但是我做不到that.P

编辑：

 today = datetime.datetime.today().weekday() + 1

营业时间Table数据

 id BID Day  Stathours EndHour        
145 1   1   12:00am 12:00am
146 1   2   Closed  Closed
147 1   3   12:00am 12:00am
148 1   4   Closed  Closed
149 1   5   12:00am 12:00am
150 1   6   12:00am 12:00am
151 1   7   12:00am 12:00am
152 2   1   12:00am 12:00am
153 2   2   12:00am 12:00am
154 2   3   12:00am 12:00am
155 2   4   12:00am 12:00am
156 2   5   12:00am 12:00am
157 2   6   12:00am 12:00am
158 2   7   12:00am 12:00am

现在我想使用 filter(BusinessHours.Day = today) 和 annotate.So 从 BusinessHours 过滤数据，输出如下。

 BID Businame ContactPerson Rating Comment StartHour  EndHours
    1    First      First        4      2    12:00am  12:00pm
    2    Second      Second      4.5     2   12:00am  12:00pm

StartHour 和 EndHour 是当前日期的给定值。

Answer 1

您可以使用 annotation with aggregation:

from django.db.models import Count, Avg

qs = Business.objcts.annotate(comment_count=Count('businesscomment'), 
                              avg_rating=Avg('businesscomment__rating'))

然后在查询集中您可以访问 comment_count 或 avg_rating:

# I am using these fields in values_list for example
qs.values_list('BID', 'BusinessName', 'ContactPerson', 'comment_count', 'avg_rating')

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