Django ORM - `select_related`

Jul 18, 2023 • 1 min read

Suppose that we have the following models:

class Engagement(models.Model):
    source_type = models.CharField(max_length=256)


class FeedbackRequest(models.Model)
    slug = models.CharField(max_length=256)
    engagement = models.OneToOneField(
        Engagement,
        on_delete=models.CASCADE
    )

If for some reason, we wanted to query the FeedbackRequest instances and within an iterator, access the engagement field, like the following snippet:

for feedback_request in FeedbackRequest.objects.all()[:15]:
    print(feedback_request.engagement)

This would make an extra query every time we call feedback_request.engagement.

To improve on this, we can use the Django ORM’s select_related method which constructs a JOIN query with the related field once rather than making the query every time we access the related field.

feedback_request_qs = (
    FeedbackRequest.objects
        .select_related('engagement')
        .all()[:15]
)
for feedback_request in feedback_request_qs:
    print(feedback_request.engagement)

So if we profile this query now, we’ll see that we’ve only made one query and all of our accesses to engagement won’t make another query.