How to Find Departments with No Volunteers Using Django's ORM LEFT JOIN?

LEFT JOIN Django ORM

Querying for Departments with No Volunteers Using ORM

In the Django Object-Relational Mapping (ORM) framework, you can use the LEFT JOIN operation to retrieve data from multiple tables, even if there are no matching rows in one of the tables.

To query for departments that have no volunteers assigned to them, you can use the following approach:

from django.db.models import OuterRef, Subquery

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

Explanation:

• The OuterRef class represents the primary key of the current model instance. In this case, OuterRef('pk') refers to the primary key of the Department model.
• The Subquery class is used to create a subquery that represents the departmentvolunteer table. The isnull filter checks whether the departmentvolunteer table has a matching row for each department.
• The filter method is used to filter out departments that have matching rows in the departmentvolunteer table. The values_list method is then used to retrieve only the name field of the matching departments.

This approach allows you to span multi-valued relationships between models and retrieve data even when there are no matches in one of the tables.

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