Django Nested Json of Multiple Foreign Key Models - django

Question

I've 4 model that has relationship through FK.
class Journal(models.Model):
name = models.CharField(max_length=255)
class Volume(models.Model):
journal = models.ForeignKey(Journal, related_name='volumes')
number = models.IntegerField()
class Issue(models.Model):
volume = models.ForeignKey(Volume, related_name='issues')
number = models.IntegerField()
class Article(models.Model):
issue = models.ForeignKey(Issue, related_name='articles')
title = models.CharField(max_length=255)
I need a JSON format like follow structure.
journal: [
{ name: 'Volume number goes here', type: 'folder', id: 'F1',
data: [
{ name: 'Issue number goes here', type: 'folder', id: 'F1F1',
data: [
{ name: 'Article name goes here>', type: 'item', id: 'F1F1I1' },
{ name: 'Article name goes here>', type: 'item', id: 'F1F1I2' },
{ name: 'Article name goes here>', type: 'item', id: 'F1F1I3' },
]},
{ name: 'Issue number goes here', type: 'folder', id: 'F1F2',
data: [
{ name: 'Article name goes here>', type: 'item', id: 'F1F2I1' },
{ name: 'Article name goes here>', type: 'item', id: 'F1F2I2' },
{ name: 'Article name goes here>', type: 'item', id: 'F1F2I3' },
]},
]
},
{ name: 'Volume number goes here', type: 'folder', id: 'F2',
data: [
{ name: 'Issue number goes here', type: 'folder', id: 'F1F1',
data: [
{ name: 'Article name goes here>', type: 'item', id: 'F2F1I1' },
{ name: 'Article name goes here>', type: 'item', id: 'F2F1I2' },
{ name: 'Article name goes here>', type: 'item', id: 'F2F1I3' },
]},
{ name: 'Issue number goes here', type: 'folder', id: 'F1F2',
data: [
{ name: 'Article name goes here>', type: 'item', id: 'F2F2I1' },
{ name: 'Article name goes here>', type: 'item', id: 'F2F2I2' },
{ name: 'Article name goes here>', type: 'item', id: 'F2F2I3' },
]},
]
}
],
I've tried several stuff but it will cause hundred of sql queries ( because of for loops )
Any ideas ?

Answer 1

You can create that JSON file using exactly 4 queries. You just have to use prefetch_related.
Here is some proof of concept (for queries counter to work, you must have DEBUG=True):
from django.db import connection
journals = Journal.objects.all().prefetch_related('volumes', 'volumes__issues', 'volumes__issues__articles')
for journal in journals:
print "%s" % journal.name
for volume in journal.volumes.all():
print " %d" % volume.number
for issue in volume.issues.all():
print " %d" % issue.number
for article in issue.articles.all():
print " %s" % article.title
print len(connection.queries)
That will print simple tree of your objects and number of queries at the end, which will equal 4 (if there wasn't any queries done before in that connection). From that there is not far to create your JSON output.
In creating that exact JSON, Django REST Framework can be helpful. Assuming that you have all your serializers done and nested, feeding JournalSerializer With above queryset, will create exactly 4 queries to database.

Answer 2

Use select_related and prefetch_related. These methods are used in the Django ORM to do SQL JOINs, so you won't be duplicating queries.