How to create 2 objects from separate models with a single serializer and also retrieve them from the database with a single serializer in Django RF?

Question

I have 3 models: Maker, Item and MakerItem that creates the relation between the items and their makers:

class Maker(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
    name = models.CharField(max_length=100)

class Item(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
    name = models.CharField(max_length=100)

class MakerItem(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
    item_id = models.ForeignKey(Item, on_delete=models.CASCADE)
    maker_id = models.ForeignKey(Maker, on_delete=models.CASCADE)

the items can have a random amount of makers.

I want to create both the Item and the MakerItem objects at the same time with a single set of data, for example if a Maker with id = "abcd" already exists, and I go to /item and send a POST request with the following data:

{
    "name": "item1",
    "makers": [
        {
            "maker_id": "abcd"
        }
    ]
}

I want the serializer to create the Item object and the MakerItem object. I have achieved this, with the following setup:

views.py

class ItemListCreate(ListCreateAPIView):
    queryset = Item.objects.all()
    serializer_class = ItemSerializer

serializers.py

class ItemSerializer(serializers.ModelSerializer):
    class MakerItemSerializer(serializers.ModelSerializer):
        class Meta:
            model = MakerItem
            exclude = ['id', 'item_id']

    makers = MakerItemSerializer(many=True)

    class Meta:
        model = Item
        fields = ['id', 'name', 'makers']

    def create(self, validated_data):
        maker_item_data = validated_data.pop('makers')
        item_instance = Item.objects.create(**validated_data)
        for each in maker_item_data:
            MakerItem.objects.create(
                item_id=check_instance,
                maker_id=each['maker_id']
            )
        return item_instance

but when Django tries to return the created object, it always gives me the error:

AttributeError at /item/

Got AttributeError when attempting to get a value for field `makers` on serializer `ItemSerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `Item` instance.
Original exception text was: 'Item' object has no attribute 'makers'.

What am I doing wrong?

Thanks

EDIT: To clarify, the objects get created and populate the database correctly, but when the browsable API that DRF provides tries to display the created object, it gives me the error above.

@Swift that will only throw another error ```AssertionError at /item/ The field 'makers' was declared on serializer ItemSerializer, but has not been included in the 'fields' option.``` — obvionaoe, Nov 23 '21 at 18:32
The serialisers aren't supposed to be declared and nested in this way. Declare both serialisers on their own — Swift, Nov 23 '21 at 18:39
This may be against best practices, but it has no effect whatsoever, it still throws the same error I pointed out in the question when separated. — obvionaoe, Nov 23 '21 at 18:47
change field name to `makers_set` ? Just guessing at this point I'm afraid! — Swift, Nov 23 '21 at 19:48

Swift · Answer 1 · 2021-11-24T14:14:06.437

0

Change:

class ItemSerializer(serializers.ModelSerializer):
    class MakerItemSerializer(serializers.ModelSerializer):
        class Meta:
            model = MakerItem
            exclude = ['id', 'item_id']

    makers = MakerItemSerializer(many=True)

To:

class ItemSerializer(serializers.ModelSerializer):
    class MakerItemSerializer(serializers.ModelSerializer):
        class Meta:
            model = MakerItem
            exclude = ['id', 'item_id']

    makers = MakerItemSerializer(many=True, source="makeritem_set")

Hope this works!

For clarity, you're attempting to serialise the reverse relationship between MakerItem and Item for this serialiser.

This means that the attribute on your object is automatically set by Django as fieldname_set but you can override this behaviour by setting the related_name kwarg on the field and then makemigrations and migrate it.

In your case you would need to do:

maker_id = models.ForeignKey(Maker, on_delete=models.CASCADE, related_name="maker_items")

And then update the field in the Meta to match the new field name, this way you don't have to manually specify source. Because actually the attribute "makers" is misleading, due to the fact its actually the MakerItem, not the Maker itself.

See https://docs.djangoproject.com/en/3.2/ref/models/relations/ for further details about this behaviour.

edited Nov 24 '21 at 14:14

answered Nov 23 '21 at 19:52

Swift

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Hi, could you clarify this a bit? I don't really understand what you mean – obvionaoe Nov 24 '21 at 00:11
This page should help: https://docs.djangoproject.com/en/3.2/ref/models/relations/ – Swift Nov 24 '21 at 08:03
@obvionaoe updated answer – Swift Nov 24 '21 at 14:15
Thanks, I ended up using the ManyToManyField! Is there a way to override the id field for the ManyToManyField? It defaults to an auto incrementing integer but I need it to use an UUID – obvionaoe Nov 28 '21 at 16:42
I wouldnt recommend it. You could certainly add a UUID field though. Have a Google, such a thing exists, I've used it! :) – Swift Nov 28 '21 at 17:51
Why not? Will it break something? Does it have performance issues? I'm using PostgreSQL – obvionaoe Nov 28 '21 at 19:03
No, you can re-assign it, but im not really sure why you would opt to re-assign the auto incrementing ID with a UUID. Are you trying to do something specifically in this way? As in, is it a design choice? If so, why? – Swift Nov 28 '21 at 22:55
all other tables use UUIDs, it makes sense to use one in this one too, I believe – obvionaoe Nov 29 '21 at 10:18
I found a way! Using the through model – obvionaoe Nov 29 '21 at 14:13
https://stackoverflow.com/a/35114415/8874154 – Swift Nov 29 '21 at 17:56

How to create 2 objects from separate models with a single serializer and also retrieve them from the database with a single serializer in Django RF?

1 Answers1