[Fixed]-Django Rest Framework nested serializer not showing related data

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.