Add Serializer on Reverse Relationship – Django Rest Framework

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', 
        )