Working With Select_Related In Django

select_related is a queryset method in Django which returns new queryset result for a Django Model. select_related is more helpful in foreign-key relationships. It is useful when we are going to use those foreign-key relationship data later in our code. select_related is a preformance booster, which caches the result for the foreign-key relationships, so that django ORM do not need to query again to get the data for the foreign-key relationships.

How select_related works?

select_related creates a single complex query using joins on related models and retrives the data at once and caches it for later use, So foreign-key relationships won't require database queries.

Example Use Case:

lets consider below models

class ClassRoom(models.Model):
    name = models.CharField(max_length=100)

    def __str__(self):
        return self.name

class Student(models.Model):
    name = models.CharField(max_length=100)
    class_room = models.ForeignKey(ClassRoom, on_delete=models.CASCADE)

    def __str__(self):
        return self.name

let's consider below queries

query1 = Student.objects.all()
for student in query1:
    print(student.name)
    print(student.class_room.name)  # queries the database for relational data
query2 = Student.objects.all().select_related()
for student in query2:
  print(student.name)
  print(student.class_room.name) # no database query

In above code when we use the query1 it generates the SQL code like below

SELECT
    "myapp_student"."id", "myapp_student"."name",
    "myapp_student"."class_room_id"
FROM
    "myapp_student"

In above code when we use the query2 it generates the complex SQL code like below

SELECT
  "myapp_student"."id", "myapp_student"."name",
  "myapp_student"."class_room_id", "myapp_classroom"."id",
  "myapp_classroom"."name"
FROM "myapp_student"
INNER JOIN "myapp_classroom" ON ("myapp_student"."class_room_id" = "myapp_classroom"."id")

In the query2 django is joining the foreign key relationships so, when foreign key data is required then django ORM will not hit the database. query1 results in 1 + number of records queries to the database where as in query2 it results in only 1 database query. So, It's recommended to use the select_related when we use foreign key related data.

Related Blog Articles

Django Model Crud Operations

Let's do CRUD operations on django model. In the previous article we have seen how to create a table or model in the django application. Let's write basic queries like insert, update and delete queries using django ORM.

Usage Of Group_by And String_agg In Django, Postgresql

Learn how to optimise the querying in django using group_by and string_agg in django postgres.