How to get count of subjects taken by each student? - django

Question

I've two models 'Students' and 'Enrollments'.
The schema for these is as below:
class Students(models.Model):
id = models.AutoField(primary_key=True, unique=True)
name = models.CharField()
class Enrollments(models.Model):
enroll_id = models.AutoField(primary_key=True, unique=True)
student_id = models.ForeignKey(Students, on_delete=models.CASCADE)
subjects = models.charField()
I'm trying to achieve the result of following SQL query in Django Rest Framework, for getting number of subjects enrolled by students (individually).
select
s.id, s.name, count(e.subjects) as count
from Students as s
left outer join Enrollments as e
on e.student_id_id = s.id
group by s.id, s.name, e.subjects
order by count asc;
This query returns result like:
---------------------------
| id | name | count |
---------------------------
| 1 | a | 1 |
| 2 | b | 0 |
| 3 | c | 2 |
---------------------------
Can anyone please help me acheive this kind of result.
Note: I need 0 count students details also.

Answer 1

What you can do is when you are creating a serializer, you can add a serializer method field which will get the count for you.
Add this at the top of your serializer:
count = serializers.SerializerMethodField('get_count')
Then add a function inside your serializer like this:
def get_count(self, obj):
try:
return Enrollments.objects.filter(student_id=obj.id).count()
except:
return None
Finally, add 'count' to your field list. You can then add as many fields as you want. I hope this will get you your desired result. Also don't forget to use "select_related" in the ORM inside your view to reduce the amount of queries.