Lets say I have 2 models with a foreign key relation which are used to bundle books:
class Bundle(models.Model):
name = models.CharField(max_length=100)
class Book(models.Model):
name = models.CharField(max_length=20)
isbn = models.CharField(max_length=13)
bundle = models.ForeignKey(Bundle)
we'll identify the bundles by concatenating the ISBN numbers with the delimiter like so:
123456788 & 123456789
To further export a list of available bundles for further processing we need that number.
I'm aware I could use:
for bundle in Bundle.objects.all():
complex_isbn = ' & '.join(bundle.book_set.all().values_list('isbn', flat=True))
But this would just be too slow for the real-world purpose. Is there a way I could use annotate to accomplish this? If so, how? I'm struggling to find my way through the docs on how to accomplish concatenating multiple foreign key entries.
CodePudding user response:
You can make use of the StringAgg
aggregate function [Django-doc], which is only available for PostgreSQL. You thus can annotate the Bundle
s with the complex ISBN:
from django.contrib.postgres.aggregates import StringAgg
Bundle.objects.annotate(
complex_isbn=StringAgg('isbn', delimiter=' & ')
)
The Bundle
s that arise from this queryset will have an extra attribute .complex_isbn
.