django query model A and exclude some items from related model B

Question

I'm new to Django and I'm facing a question to which I didn't an answer to on Stackoverflow. Basically, I have 2 models, Client and Order defined as below:

class Client(models.Model): name = models.CharField(max_length=200) registration_date = models.DateTimeField(default=timezone.now) # .. class Order(models.Model): Client = models.ForeignKey(ModelA, on_delete=models.CASCADE, related_name='orders') is_delivered = models.BooleanField(default=False) order_date = models.DateTimeField(default=timezone.now) # ..

I would like my QuerySet clients_results to fulfill the 2 following conditions:

Client objects fill some conditions (for example, their name start with "d" and they registered in 2019, but it could be more complex)
Order objects I can access by using the orders relationship defined in 'related_name' are only the ones that fills other conditions; for example, order is not delivered and was done in the last 6 weeks.

I could do this directly in the template but I feel this is not the correct way to do it. Additionally, I read in the doc that Base Manager from Order shouldn't be used for this purpose. Finally, I found a question relatively close to mine using Q and F, but in the end, I would get the order_id while, ideally, I would like to have the whole object.

Could you please advise me on the best way to address this need?

Thanks a lot for your help!

willeM_ Van Onsem · Accepted Answer · 2019-10-21 21:49:36Z

You probably should use a Prefetch(..) object [Django-doc] here to fetch the related non-delivered Orders for each Client, and stores these in the Clients, but then in a different attribute, since otherwise this can generate confusion.

You thus can create a queryset like:

from django.db.models import Prefetch from django.utils.timezone import now from datetime import timedelta last_six_weeks = now() - timedelta(days=42) clients_results = Client.objects.filter( name__startswith='d' ).prefetch_related( Prefetch( 'orders', Order.objects.filter(is_delivered=False, order_date__gte=last_six_weeks), to_attr='nondelivered_orders' ) )

This will contain all Clients where the name starts with 'd', and each Client object that arises from this queryset will have an attribute nondelivered_orders that contains a list of Orders that are not delivered, and ordered in the last 42 days.

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