django left join with where 子句子表达式

Question

我目前正在尝试使用 Django (v1.10) ORM 做一些我认为应该可行的方法，但我很难理解如何应用记录的方法来解决我的问题。

编辑：所以这是我一起破解的 sql，以从 dbshell 返回我想要的数据，现在使用 postgresql 数据库，在我意识到我原来的 sqlite3支持的 sql 查询不正确：

select 
    voting_bill.*,vv.vote 
from 
    voting_bill 
left join 
    (select 
        voting_votes.vote,voting_votes.bill_id 
    from 
        voting_bill 
    left join 
        voting_votes 
    on 
        voting_bill.id=voting_votes.bill_id 
    where 
        voting_votes.voter_id = (select id from auth_user where username='richard' or username is Null)
   ) 
as 
    vv 
on 
    voting_bill.id=vv.bill_id;

这是我的投票应用程序的“models.py”：

from django.db import models
from django.contrib.auth.models import User


class Bill(models.Model):
    name = models.CharField(max_length=255)
    description = models.TextField()
    result = models.BooleanField()
    status = models.BooleanField(default=False)

    def __str__(self):
        return self.name


class Votes(models.Model):
    vote = models.NullBooleanField()
    bill = models.ForeignKey(Bill, related_name='bill',
                             on_delete=models.CASCADE,)
    voter = models.ForeignKey(User, on_delete=models.CASCADE,)

    def __str__(self):
        return '{0} {1}'.format(self.bill, self.voter)

我可以看到我的 sql 按我预期的方式工作，投票被添加到末尾，或者如果用户尚未投票，则返回 null。

我正在努力使查询集采用这种格式，以便我可以在模板中对其进行迭代以生成一个表，如果结果为空，我可以提供一个链接，将用户带到另一个视图。

我已阅读有关 select_related 和 prefetch_related 的信息，但正如我所说，我正在努力弄清楚如何将其转换为如何在 SQL 中执行此操作。

Answer 1

希望我正确理解了您的问题。试试这个：

votes = Votes.objects.filter(voter__username='django').select_related('bill')

你可以使用它。但我认为在这种情况下你不需要select_related。

bills_for_user = Bill.objects.filter(votes__voter__username='django').select_related('votes').distinct()

现在您可以迭代您的 bills_for_user

for bill in bills_for_user:
    bill_name = bill.name
    bill_description = bill.description
    bill_result = bill.result
    bill_status = bill.status

    # and there are several variants what you can with votes
    bill_votes = bill.votes_set.all()  # will return you all votes for this bill
    bill_first_vote1 = bill.votes_set.first() # will return first element in this query or None if its empty
    bill_first_vote2 = bill.votes_set.all()[0] # will return first element in this query or Error if its empty
    bill_last_vote = bill.votes_set.last()[0] # will return last element in this query or None if its empty
    # you can also filter it for example by voting
    bill_positive_votes = bill.votes_set.filter(vote=True) # will return you all votes for this bill with 'vote' = True
    bill_negative_votes = bill.votes_set.filter(vote=False) # will return you all votes for this bill with 'vote' = False
    bill_neutral_votes = bill.votes_set.filter(vote=None) # will return you all votes for this bill with 'vote' = None