Django Rest Framework nested serializer not showing related data

Question

I have a basic setup using the Django Rest Framework. I have two models and a nested serializer setup:

# models.py

from django.db import models

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

    def __unicode__(self):
        return u'%s' % (self.name)

class Group(models.Model):
    plan = models.ForeignKey('plan')
    name = models.CharField(max_length='50')
    weight = models.SmallIntegerField()

    def __unicode__(self):
        return u'%s - %s' % (self.name, self.plan.name)


# serializer.py

from plans.models import Plan, Group
from rest_framework import serializers

class GroupSerializer(serializers.ModelSerializer):
    class Meta:
        model = Group
        fields = ('name', 'weight')

class PlanSerializer(serializers.ModelSerializer):
    group = GroupSerializer(many=True, read_only=True)

    class Meta:
        model = Plan
        fields = ('name', 'group')


# views.py

from rest_framework import viewsets

from plans.models import Plan
from plans.serializers import PlanSerializer

class PlanViewSet(viewsets.ModelViewSet):
    queryset = Plan.objects.all()
    serializer_class = PlanSerializer

When I view the serializers relationships in Django's Shell it shows the relationship correctly:

PlanSerializer():
name = CharField(max_length='100')
group = GroupSerializer(many=True, read_only=True):
    name = CharField(max_length='50')
    weight = IntegerField()

What I end up getting back via cURL is:

[
    {
        name: Test Plan
    }
]

What I expect to get back is:

[
    {
        name: Test Plan,
        group: [
                {
                    name: Test Group,
                    weight: 1
                }
        ] 
    }
]

There is no nested data coming through. I'm at a lose for what I've not setup correctly here. Can anyone point me in the correct direction?

Todor Todor · Accepted Answer · 2015-04-15T14:09:59

The problem comes from your queryset: queryset = Plan.objects.all(). None of the items in this queryset has .group attribute that's why your result is empty. By default Django creates a reverse relation of the plan ForeignKey called group_set (unless you don't rename it via related_name) (this means that every plan item in the queryset have a group_set attribute which is a queryset containing all the groups of this plan). You can use this attribute in order to get a proper serialization. This means to change:

class PlanSerializer(serializers.ModelSerializer):
    group_set = GroupSerializer(many=True, read_only=True)

    class Meta:
        model = Plan
        fields = ('name', 'group_set')

If you really want to stick with group (btw this is a very bad name for a list of groups). You can hack it with prefetch_related like so:

queryset = Plan.objects.prefetch_related('group_set', to_attr='group')

this way every plan item will have a group attribute - a queryset containing all the groups for this plan.

Django Rest Framework nested serializer not showing related data

2 Answers