Related field not populating with data in django rest framework serializer

Question

Have two models:

class Topic(Base):
    name = models.CharField(max_length=100, unique=True)
    display_name = models.CharField(max_length=100)

class SubTopic(Base):
    name = models.CharField(max_length=100, unique=True)
    display_name = models.CharField(max_length=100)
    topic = models.ForeignKey(Topic, on_delete=models.CASCADE)

and have two serializers:

class SubTopicSerializer(serializers.ModelSerializer):

    class Meta:
        model = SubTopic
        fields = ('topic', 'name', 'display_name')


class TopicSerializer(serializers.ModelSerializer):
    sub_topics = SubTopicSerializer(many=True, read_only=True)

    class Meta:
        model = Topic
        fields = ('id', 'display_name', 'sub_topics')

the subtopics are not populating in the response.

Answer 1

I was missing the related_name parameter on the ForeignKey.

class SubTopic(Base):
    name = models.CharField(max_length=100, unique=True)
    display_name = models.CharField(max_length=100)
    topic = models.ForeignKey(ContextualTopic, related_name="sub_topics", on_delete=models.CASCADE)