How can small queries be combined in django rest framework with nested serializers?

Question

I have a database relationship like the following:

  +-----------+
  | Container |
  +-----------+
  | id        |
  +-----------+
        | 
        |1..n 
+--------------+
| Child        |
+--------------+
| id           |
| container_id |
+--------------+

I want to nest the serializers for the Child in the Container serializer so I can get both objects in one HTTP request. However, when I query for a list of Containers, Django REST Framework does a single query to get all the Containers, but then does a bunch of individual queries for each set of Child objects associated with a specific Container object. Here is my toy example:

# Models
class Container(models.Model):
    pass

class Child(models.Model):
    container = models.ForeignKey(Container,related_name="children")

# Serializers
class ChildSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Child
        fields = ('url','container')

class ContainerSerializer(serializers.HyperlinkedModelSerializer):
    children = ChildSerializer(many=True)
    class Meta:
        model = Container
        fields = ('url','children')

# Views
class ContainerViewSet(viewsets.ModelViewSet)
    queryset = Container.objects.all()
    serializer_class = ContainerSerializer

class ChildViewSet(viewsets.ModelViewSet)
    queryset = Child.objects.all()
    serializer_class = ChildSerializer

Is there a way to coalesce all the Child queries into one backend query and then distribute the results amongst the Container objects?

score 1 · Accepted Answer · edited May 23 '17 at 12:27

Django REST Framework will not optimize your queries for you, it's up to you to decide how best to remove any N+1 queries. You should follow the guidelines covered in the Django documentation to handle performance issues.

In the case of ForeignKey relationships, you should use select_related in your query, which will pre-fetch the objects in the original query.

In the case of ManyToMany and GenericForeignKey relationships, you should use prefetch_related. I've written quite a bit about this in another Stack Overflow answer, but the gist is that you use it similar to select_related.

You should override the query in get_queryset on the view for best results, as you don't need to worry about Django REST Framework incorrectly cloning the queryset when used as an attribute on the class.

How can small queries be combined in django rest framework with nested serializers?

1 Answers1