How to Find Unassigned Departments in Django Using LEFT JOIN?

Discovering Unassigned Departments with Django ORM: LEFT JOIN

In Django, you can elegantly execute LEFT JOIN operations to retrieve specific data. Let's explore how to use this technique to identify departments without assigned volunteers.

LEFT JOIN with Django ORM

Given the following models:

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


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

We can leverage Django ORM's LEFT JOIN to obtain departments with no associated volunteers:

qs = Department.objects.filter(departmentvolunteer__isnull=True).values_list('name', flat=True)

Exploring the Query

The generated query is:

SELECT "app_department"."name" 
FROM "app_department" LEFT OUTER JOIN "app_departmentvolunteer" 
ON ( "app_department"."id" = "app_departmentvolunteer"."department_id" )
WHERE "app_departmentvolunteer"."id" IS NULL

This query retrieves department names where there aren't any entries in the DepartmentVolunteer table.

Spanning Multi-Valued Relationships

Django ORM provides support for "Spanning Multi-Valued Relationships" through filtering, which allows for efficient traversal of complex relationships.

The documentation on this topic can be found here: https://docs.djangoproject.com/en/stable/topics/db/queries/#spanning-multi-valued-relationships

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