Django左加入?

问题描述

我大概有这样的模型:

class ModelA(models.Model):
    field = models.CharField(..)

class ModelB(models.Model):
    name = models.CharField(.., unique=True)
    modela = models.ForeignKey(ModelA, blank=True, related_name='modelbs')

    class Meta:
        unique_together = ('name','modela')

我想做一个查询，内容如下:获取所有ModelA，其中字段名称等于X，其中ModelB模型的名称为X或根本没有模型名称"

I want to do a query that says something like: "Get all the ModelA's where field name equals to X that have a ModelB model with a name of X OR with no model name at all"

到目前为止，我有这个:

So far I have this:

ModelA.objects.exclude(field=condition).filter(modelsbs__name=condition)

这将为我提供至少具有一个modelB的所有ModelA(实际上，它始终只是一个)-但是，如果ModelA没有相关的ModelB，则它将不在结果集中.我需要它放在结果集中，像obj.modelb = None

This will get me all the ModelAs that have at least one modelB (and in reality it will ALWAYS be just one) - but if a ModelA has no related ModelBs, it will not be in the result set. I need it to be in the resultset with something like obj.modelb = None

我该怎么做?

推荐答案

使用Q组合两个条件:

from django.db.models import Q
qs = ModelA.objects.exclude(field=condition)
qs = qs.filter(Q(modelbs__name=condition) | Q(modelbs__isnull=True))

要检查生成的SQL查询:

To examine the resulting SQL query:

print qs.query.as_sql()

在类似的查询上，这将生成一个LEFT OUTER JOIN ... WHERE(a.val = b或a.id IS NULL).

On a similar query, this generates a LEFT OUTER JOIN ... WHERE (a.val = b OR a.id IS NULL).

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