Django Rest Framework get many-to-many relation showing in API

Question

My little podcast backend has some models, one of which is Episode. Now there are two new ones for guests (Guest) and topics (Topic) - both of which have a ManyToManyField to Episode. This basically works as intended in the admin. Now I need to bring that to the API.

The detailed Serializer/View for each episode already has some other related data (no many-to-many so far). I also checked this post but still can't get it it work and think I might miss something.

Here's the essence just for the guest example to keep it dense, topic is literally identical:

Episode Model:

class Episode(models.Model):
    number          = models.CharField(max_length=10, blank=False)
    title           = models.CharField(max_length=100, blank=False)
    show            = models.ForeignKey(Show, on_delete=models.PROTECT)
    created_at      = models.DateTimeField(auto_now=False)
    published_at    = models.DateTimeField(auto_now=False)
    updated_at      = models.DateTimeField(auto_now=False)
    cover_image     = models.URLField(null=True)

    def __str__(self):
        return self.title

Guest Model:

class Guest(models.Model):
    episodes        = models.ManyToManyField(Episode)
    name            = models.CharField(max_length=100, blank=False)
    twitter         = models.CharField(max_length=20, null=True)

    def __str__(self):
        return self.name

Serializer:

class GuestSerializer(serializers.ModelSerializer):
    class Meta:
        model = Guest
        fields = ('name',)

Episode Serializer where it should appear:

class EpisodeDetailSerializer(serializers.ModelSerializer):
    ...
    guest = GuestSerializer(many=True, read_only=True)
    topic = TopicSerializer(many=True, read_only=True)

    class Meta:
        model = Episode
        fields = (..., 'guest', 'topic')
        depth = 1

I have put some data on for guests and topics but I can't get them showing up on the API. I've also tried 'Guest.episodes.through' like I do have it on the admin for Inline Admin Classes but that didn't change anything.

Can you post your Episode model as well, since that's the one on which your last serializer is based? And do you have `related_names` set for any of these m2m relationships? — souldeux, Jun 01 '18 at 02:41
@souldeux I just did add the Episode model. No, I didn't set `related_names` so far. I read they are optional? — Helmi, Jun 01 '18 at 05:09

score 2 · Accepted Answer · answered Jun 01 '18 at 12:57

Your Episode model does not have any forward relationships to your Guest or Topic models. For that reason, your Episode serializer should seek to serialize guest_set and topic_set instead of guest and topic.

If you specify a related name for those M2M field, you can customize the naming convention here. Right now you have this:

class Guest(models.Model):
    episodes = models.ManyToManyField(Episode)
    ...

Which means that to get to all the Guests associated with a given Episode called ep, the reverse lookup would be ep.guest_set.all(). Define a related name and you can call the middle term whatever you want:

class Guest(models.Model):
    episodes = models.ManyToManyField(Episode, related_name='guests')
    ...

With this you could use ep.guests.all(), and you could update your Episode serializer to look for guests as opposed to guest_set.

Django Rest Framework get many-to-many relation showing in API

1 Answers1