Group the django object according to the foreign key

Question

I want to group the model object according to the foreign key with a query or without any iteration.

example: Let, I have a model student:

class Student(models.Model):
    id=models.AutoField(primary_key=True)
    parentId=models.ForeignKey(parent)
    name=models.CharField()
    age=models.IntegerField()

Here I want to create the group the student object with respect to the parentId using a query or any optimized way.

Possible duplicate of [How to query as GROUP BY in django?](https://stackoverflow.com/questions/629551/how-to-query-as-group-by-in-django) — rje, Oct 18 '18 at 08:28
@rje: this does not make a list of lists, but a queryset of dictionaries. The linked answer has as typical use case to make aggregates *per* "group". — Willem Van Onsem, Oct 18 '18 at 08:29
Yes. I want to achieve this using Django ORM query if possible. @zypro — GAJESH PANIGRAHI, Oct 18 '18 at 08:35
@GAJESHPANIGRAHI probably you can write down a SQL Statement which solves the problem? — zypro, Oct 18 '18 at 08:46

score 2 · Accepted Answer · edited Oct 18 '18 at 10:49

2

Try using a defaultdict

    from collections import defaultdict

    students = defaultdict(list)
    for result in Student.objects.order_by('parentId'):
        students[result.parentId].append(result)

edited Oct 18 '18 at 10:49

GAJESH PANIGRAHI

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answered Oct 18 '18 at 08:39

zypro

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Thanks, @zypro !! – GAJESH PANIGRAHI Oct 18 '18 at 10:00
@zypro Helped me a lot! Is there something else we can with the newer versions? – Vikram Bankar Jul 06 '21 at 04:36
@VikramBankar Well since python 3.9 you can use the merge (|) and update (|=) operators for defaultdicts too. In the upper case this might not help you very much. ;-) – zypro Jul 07 '21 at 05:46

score 1 · Answer 2 · answered Oct 18 '18 at 08:26

If you order the elements by the foreign key, then you can use itertools.groupby for this:

from itertools import groupby
from operator import attrgetter

result = [
    list(v)
    for __, v in grouby(Student.objects.order('parentId_id'), attrgetter('parentId_id'))
]

or if you want to make a dictionary for where you map parent objects to a list of Students:

result = {
    k: list(v)
    for k, v in grouby(
        Student.objects.prefetch_realted('parentId').order('parentId_id'),
        attrgetter('parentId')
    )
}

This will thus generate a list of lists, where in each sublist, students with the same parentId are grouped together.

Note that ForeignKeys normally have no _id, or Id suffix. So you might want to change the model to:

class Student(models.Model):
    parent = models.ForeignKey()
    name = models.CharField()
    age = models.IntegerField()

Group the django object according to the foreign key

2 Answers2