How to optimize a Django query involving multiple models

Data and Databases
1

I am currently working with three Django models and need assistance in improving the efficiency of my queries.

class ShipmentPPTLMapping(models.Model):
    pptl_id = models.CharField(max_length=255)
    shipment_id = models.ForeignKey('Shipment', related_name='pptls')

class ShipmentBagSealMapping(models.Model):
    bag_seal = models.CharField(max_length=255)
    status = models.CharField(max_length=255, default='open')
    shipment_id = models.ForeignKey('Shipment', related_name='bags')

class Shipment(models.Model):
    job_id = models.CharField(max_length=255)

I need to retrieve all bags from ShipmentBagSealMapping that have a status of ‘closed’ based on a specific pptl_id from ShipmentPPTLMapping.

Currently, I am using multiple queries and inner loops:

shipment_pptl_mapping_list = ShipmentPPTLMapping.objects.filter(pptl_id=pptl_id)

for shipment_mapping in shipment_pptl_mapping_list:
    closed_bags = shipment_mapping.shipment_id.bags.filter(status='closed')
    for bag in closed_bags:
        # process each bag
        pass

This results in numerous database queries, and I am looking to streamline this to a single query. Is it possible to get all the closed bags directly without needing to loop through the ShipmentPPTLMapping items first?

I am aiming for something resembling:

closed_bags = OptimizedQuery
for bag in closed_bags:
    # process bag
    pass

What is the most effective way to merge these relationships into one optimized Django query?

2

Use double underscores to join the relationships in one query. Try ShipmentBagSealMapping.objects.filter(shipment_id__pptls__pptl_id=pptl_id, status='closed') - Django handles the join automatically and you’ll get all closed bags in a single DB hit.

3

Alex_Dynamo’s approach is solid, but you should add select_related() or prefetch_related() for better performance. Since you’re dealing with foreign keys, select_related('shipment_id') will do a SQL join and grab the related shipment data in one query instead of hitting the database multiple times. Your query becomes: ShipmentBagSealMapping.objects.filter(shipment_id__pptls__pptl_id=pptl_id, status='closed').select_related('shipment_id'). This really matters when you’re processing tons of data and need shipment details during bag processing.

4

Nice approach! Have you tried using annotate() with Count() to check for multiple shipments per pptl? Also, how big is your dataset - hundreds or thousands of bags? Query optimization changes depending on scale.