Add Serializer on Reverse Relationship - Django Rest Framework

Question

I have a Cart model and a CartItem model. The CartItem model has a ForeignKey to the Cart model.

Using Django Rest Framework I have a view where the API user can display the Cart, and obviously then I want to include the CartItem in the respone.

I set up my Serializer like this:

class CartSerializer(serializers.ModelSerializer):
    user = UserSerializer(read_only=True)
    cartitem_set = CartItemSerializer(read_only=True)
    class Meta:
        model = Cart
        depth = 1
        fields = (
            'id', 
            'user', 
            'date_created', 
            'voucher', 
            'carrier', 
            'currency', 
            'cartitem_set', 
        )

My problem is the second line, cartitem_set = CartItemSerializer(read_only=True).

I get AttributeErrors saying 'RelatedManager' object has no attribute 'product'. ('product' is a field in the CartItem model. If I exclude product from the CartItemSerializer I just get a new AttributeError with the next field and so on. No matter if I only leave 1 or all fields in the Serializer, I will get a error.

My guess is that for some reason Django REST Framework does not support adding Serializers to reverse relationships like this. Am I wrong? How should I do this?

PS

The reason why I want to use the CartItemSerializer() is because I want to have control of what is displayed in the response.

Solution 1

Ahmed Hosny was correct in his answer. It required the many parameter to be set to True to work.

So final version of the CartSerializer looked like this:

class CartSerializer(serializers.ModelSerializer):
    cartitem_set = CartItemSerializer(read_only=True, many=True) # many=True is required
    class Meta:
        model = Cart
        depth = 1
        fields = (
            'id', 
            'date_created', 
            'voucher', 
            'carrier', 
            'currency', 
            'cartitem_set', 
        )

Solution 2

It's important to define a related name in your models, and to use that related name in the serializer relationship:

class Cart(models.Model):
   name = models.CharField(max_length=500)

class CartItem(models.Model):
   cart = models.ForeignKey(Cart, related_name='cart_items')
   items = models.IntegerField()

Then in your serializer definition you use those exact names:

class CartSerializer(serializers.ModelSerializer):
    cart_items = CartItemSerializer(read_only=True)
    class Meta:
       model = Cart
       fields = ('name', 'cart_items',)

Comments

[Ahmed Hosny]

try cartitem_set = CartItemSerializer(read_only=True, many=True)

[Ranvijay Sachan]

can we change name cartitem_set to items ?

[Aaron C. de Bruyn]

You can change the name by doing something like: items = CartItemSerializer(read_only=True, many=True, source='cartitem_set')

[Gonzalo]

You can change cartitem_set name, from related_name. In class CartItem : cart = models.ForeignKey(Cart, related_name='items') , then in "fields" inside serializer, you must use name items

[Shahabaz]

I have an issue here. I am not getting any column as cartitem_set in the JSON response any idea? (Data is present). Response returns without cartitem_set key pair .

[Helen]

Is it possible to use this solution then filter the items returned in the set?

[Marcus Lind]

@Helen you can pass in a source kwarg to the field and set a custom source if you want.

[Helen]

What would the syntax of that look like? I've currently got source set to source='cartitem_set' to change the name, but since it's a string, how would I pass a filter or a queryset into it? Appreciate your help Marcus.

[adrian oviedo]

In the original question he keeps the implicit related name: cartitem_set

[Joel G Mathew]

Yes, this is very important for reverse relationship to work

[Thorvald]

This solution is incredibly slow for bigger queries, any way to increase the performance here?

[Thorvald]

Had to use a prefetch_related on the query of course: prefetch_related('cartitem_set')