Friday, February 2, 2018

Optimal query for related names in onetomany or manytomany using Django Queryset

Hi,

I am trying to optimise Django queries on my ecommerce website. One of the fundamental query is where I have no clue how to make efficient. It could be trivial and probably been known long time back. But I am new to Django and would appreciate any help. This is primarily for one to many or many to many relations.

Following is an example scenario:
(Please pardon my syntax as I want to put across the concept and not the exact django code unless it's really needed):

Model A:

class A(models.Model):
    # Fields of model A

Model B (which is related to A with foreign key):

class B(models.Model):
    a = models.ForeignKey('A', related_name='bs')

Now I would like to find out all As for which there is atleast one b. The only way I know is as follows:

A.objects.filter(bs__isnull=False)

But the above isn't an optimal way as with large of records in A and B, the above takes lot of time. It gets more inefficient if it's a many to many relationship.

Could anyone please let me know the most efficient way to use django queryset for the above scenario?

Thanks.

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