How Can I Efficiently Query for Departments with No Assigned Volunteers in Django Without Raw SQL?

LEFT JOIN Django ORM: An Alternative to Raw SQL Queries

Question:

Consider the following models:

class Volunteer(models.Model):
    ...

class Department(models.Model):
    ...

class DepartmentVolunteer(models.Model):
    ...

To query for all departments with no assigned volunteers, one can use the following SQL statement:

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 performing this query, or should one stick with raw SQL?

Answer:

Django provides a more concise and expressive way to conduct the same query using the filter() method:

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

This query spans the multi-valued relationship between Department and DepartmentVolunteer, as described in the Django documentation: https://docs.djangoproject.com/en/stable/topics/db/queries/#spanning-multi-valued-relationships

The resulting query generated by Django 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 approach eliminates the need for raw SQL and provides a more readable and maintainable query.

The above is the detailed content of How Can I Efficiently Query for Departments with No Assigned Volunteers in Django Without Raw SQL?. For more information, please follow other related articles on the PHP Chinese website!