如何在 django 中进行内连接？

我想在 Html 中显示出版物的城市、州和国家的名称。但它们位于不同的表中。

这是我的模型.py

class country(models.Model):
    country_name = models.CharField(max_length=200, null=True)
    country_subdomain = models.CharField(max_length=3, null=True)
    def __str__(self):
        return self.country_name

class countrystate(models.Model):
    state_name = models.CharField(max_length=200, null=True)
    country = models.ForeignKey(country, on_delete=models.CASCADE, null=True)
    importance = models.IntegerField(null=True)
    def __str__(self):
        return self.state_name

class city(models.Model):
    city_name = models.CharField(max_length=200, null=True)
    countrystate = models.ForeignKey(countrystate, on_delete=models.CASCADE, null=True)
    def __str__(self):
        return self.city_name

class publication(models.Model):
    user = ForeignKey(users, on_delete=models.CASCADE, null=False)
    title= models.CharField(max_length=300, null=True)
    country=models.ForeignKey(country, on_delete=models.CASCADE, null=True)
    countrystate=models.ForeignKey(countrystate, on_delete=models.CASCADE, null=True)
    city=models.ForeignKey(city, on_delete=models.CASCADE, null=True)

    def __str__(self):
        return self.title

这是我的观点.py

def publications(request):
    mypublications = publication.objects.filter(user_id=request.session['account_id'])
    dic.update({"plist": mypublications })
    return render(request, 'blog/mypublications.html', dic)

在 django 视图中，下一个 sql 查询相当于什么？

SELECT p.user_id, p.title, c.cuntry_id, c.country_name, s.state_id, s.state_name, y.city_id, y.city_name FROM publication AS p
INNER JOIN country AS c ON c.id = p.country_id
INNER JOIN countrystate AS s ON s.id = p.countrystate_id
INNER JOIN city AS y ON y.id = p.city_id

您可能正在寻找select_related https://docs.djangoproject.com/en/2.2/ref/models/querysets/#select-related，这是实现这一目标的自然方法：

pubs = publication.objects.select_related('country', 'country_state', 'city')

您可以通过以下方式检查生成的 SQLstr(pubs.query)，这应该会产生如下所示的输出（示例来自 postgres 后端）：

SELECT "publication"."id", "publication"."title", ..., "country"."country_name", ...  
FROM "publication" 
INNER JOIN "country" ON ( "publication"."country_id" = "country"."id" ) 
INNER JOIN "countrystate" ON ( "publication"."countrystate_id" = "countrystate"."id" ) 
INNER JOIN "city" ON ( "publication"."city_id" = "city"."id" ) 

然后，返回的游标值将转换为适当的 ORM 模型实例，以便当您循环这些发布时，可以通过相关表自己的对象访问相关表的值。然而，沿着预先选择的前向关系的这些访问不会导致额外的数据库命中：

{% for p in pubs %}
     {{ p.city.city_name}}  # p.city has been populated in the initial query
     # ...
{% endfor %}