Use of select_related in simple query in django

Question

I have a model in Django in which a field has a fk relationship with the teacher model. I have came across select_related in django and want to use it in my view. However, I am not sure whether to use it in my query or not.

My models:

class Teacher(models.Model):
    name = models.OneToOneField(max_length=255, default="", blank=True)
    address = models.CharField(max_length=255, default="", blank=True)
    college_name = models.CharField(max_length=255, default="", blank=True)



class OnlineClass(models.Model):
    teacher = models.ForeignKey(Teacher,on_delete=models.CASCADE)

My view:

def get(self, request,*args, **kwargs):
   
    teacher = self.request.user.teacher
    classes = Class.objects.filter(teacher=teacher) #confusion is here..............
    serializer_class = self.get_serializer_class()
    serializer = serializer_class(classes,many=True)
    return Response(serializer.data,status=status.HTTP_200_OK)

I have commented on the line or the section of the problem. So I wanted to list all the classes of that teacher. Here I have used filter. But can we use select_related here?? What I understood is if I want to show another fields of teacher model as well, for eg name or college_name, then I have to use it. Otherwise the way I have done it is correct. Also, select_related is only used for get api not for post api, is that correct??

yagus · Accepted Answer · 2021-11-02T13:51:41.943

select_related is used to select additional data from related objects when the query is executed. It results in a more complex query. But it boosts performance if you have to access related data, since no additional database queries will be required.

See documentation here.

In your code it would be possible to use select_related, but it would be inefficient, because you're not accessing related objects of the queried classes. So using select_related would result in a more complex query without any advantage.

If you wanted to use select_related, the syntax would be classes = Class.objects.select_related('teacher').filter(teacher=teacher)

score 1 · Answer 2 · answered Nov 02 '21 at 11:25

First, the easiest way to get all classes per teacher is by using the related_name attribute (https://docs.djangoproject.com/en/3.2/ref/models/fields/#django.db.models.ForeignKey.related_name).


class OnlineClass(models.Model):
    teacher = models.ForeignKey(
       Teacher,
       on_delete=models.CASCADE, 
       related_name='classes'
    ) 


# All classes of a teacher
teacher.classes.all()

When select_related is used, new sql joins are added to the Django internals SQL query. It is useful to reduce the workload in the database engine, getting the data quickly, and yes, is only for reading.


for obj in OnlineClass.objects.all():
   # This hits the database every cycle to get the teacher data,
   # with a new query like: select * from teacher_table where id = ...
   print(obj.teacher) 


for obj in OnlineClass.objects.select_related('teacher').all():
   # This don'ts hits the database. 
   # Previously, the Django ORM joined the 
   # OnlineClass and Teacher data with a single SQL query.
   print(obj.teacher)

I think that, in your example, with only one teacher, using "select_related" or not don't make big difference.

If I can do `teacher.classes.all()` ... i can use it as a property under model instead of writing get api for only this (getting all classes of a teacher) — Reactoo, Nov 02 '21 at 11:31
`classes` is property that returns a `RelatedManager`. Their docs are here: https://docs.djangoproject.com/en/3.2/ref/models/relations/#django.db.models.fields.related.RelatedManager — Tonio, Nov 02 '21 at 13:51

Use of select_related in simple query in django

2 Answers2