左加入 Django ORM [英] LEFT JOIN Django ORM

问题描述

我有以下型号:

class Volunteer(models.Model):first_name = models.CharField(max_length=50L)last_name = models.CharField(max_length=50L)email = models.CharField(max_length=50L)性别=models.CharField(max_length=1，选择=GENDER_CHOICES)班级部门(models.Model):name = models.CharField(max_length=50L, unique=True)监督者 = models.ForeignKey(志愿者，空白=真，空=真)location = models.CharField(max_length=100L, null=True)类部门志愿者(models.Model):志愿者=models.ForeignKey(志愿者)部门=models.ForeignKey(部门)助手 = 模型.BooleanField(默认=假)keyman = models.BooleanField(默认=假)船长 = models.BooleanField(默认=假)location = models.CharField(max_length=100L, blank=True, null=True)

我想查询所有没有分配志愿者的部门.我可以使用以下查询来做到这一点:

SELECTd. 姓名从vsp_department AS d左加入 vsp_departmentvolunteer AS dvON d.id = dv.department_id在哪里dv.department_id 为空；

是否有更像 django 的方式来做这件事，还是我应该使用原始 sql?

解决方案

您可以通过在查找中遵循向后关系来做到这一点.

<预><代码>>>>qs = Department.objects.filter(departmentvolunteer__isnull=True).values_list('name', flat=True)>>>打印(qs.query)SELECT "app_department"."name" FROM "app_department" LEFT OUTER JOIN"app_departmentvolunteer" ON ( "app_department"."id" = "app_departmentvolunteer"."department_id" )WHERE "app_epartmentvolunteer"."id" 为空

以下是有关跨越多值关系"查询的文档:https://docs.djangoproject.com/en/stable/topics/db/queries/#spanning-multi-valued-relationships

I have the following models:

class Volunteer(models.Model):
    first_name = models.CharField(max_length=50L)
    last_name = models.CharField(max_length=50L)    
    email = models.CharField(max_length=50L)
    gender = models.CharField(max_length=1, choices=GENDER_CHOICES)


class Department(models.Model):
    name = models.CharField(max_length=50L, unique=True)
    overseer = models.ForeignKey(Volunteer, blank=True, null=True)
    location = models.CharField(max_length=100L, null=True)


class DepartmentVolunteer(models.Model):
    volunteer = models.ForeignKey(Volunteer)
    department = models.ForeignKey(Department)
    assistant = models.BooleanField(default=False)
    keyman = models.BooleanField(default=False)
    captain = models.BooleanField(default=False)
    location = models.CharField(max_length=100L, blank=True, null=True)

I want to query for all departments that have no volunteers assigned to them. I can do so using the following query:

SELECT 
    d.name 
FROM   
    vsp_department AS d
LEFT JOIN vsp_departmentvolunteer AS dv
ON d.id = dv.department_id  
WHERE
    dv.department_id IS NULL;

Is there a more django-like way of doing this or should i just go with raw sql?

解决方案

You can do this by following the backwards relation in the lookup.

>>> qs = Department.objects.filter(departmentvolunteer__isnull=True).values_list('name', flat=True)
>>> print(qs.query)
SELECT "app_department"."name" FROM "app_department" LEFT OUTER JOIN
"app_departmentvolunteer" ON ( "app_department"."id" = "app_departmentvolunteer"."department_id" )
WHERE "app_epartmentvolunteer"."id" IS NULL

Here are the docs on queries "Spanning multi-valued relationships": https://docs.djangoproject.com/en/stable/topics/db/queries/#spanning-multi-valued-relationships

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