hanleyhansen hanleyhansen - 5 months ago 42
SQL Question

LEFT JOIN Django ORM

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
vsp_department.name
FROM
vsp_department
LEFT JOIN vsp_departmentvolunteer ON vsp_department.id = vsp_departmentvolunteer.department_id
WHERE
vsp_departmentvolunteer.department_id IS NULL;


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

Answer

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