DRF - M2M 通过 ImageField 的序列化程序提供 UnicodeDecodeError

Question

I want to serialize a model using M2M through relation. Its working fine for all other fields except ImageField . Below are my model and serializer files:

models.py

class Product(models.Model):
    name = models.CharField('Name', max_length=255, null=True, blank=True)
    description = models.TextField('Description', max_length=1000, null=True, blank=True)
    price = models.IntegerField('Price', default=0)
    image = models.ImageField('Product Image', null=True, blank=True)


class Cart(models.Model):
    user = models.CharField('User ID', default="1000", max_length=255)
    items = models.ManyToManyField("Product", through='CartActions', blank=True)
    modified = models.DateField('Last Modified')


class CartActions(models.Model):
    product = models.ForeignKey('Product', on_delete=models.CASCADE)
    cart = models.ForeignKey('Cart', on_delete=models.CASCADE)
    quantity = models.PositiveSmallIntegerField(default=0)

serializers.py

class ProductSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Product
        fields = ['id', 'name', 'description', 'price', 'image']


class CartSerializer(serializers.HyperlinkedModelSerializer):
    items = CartActionsSerializer(source='cartactions_set', many=True)

    class Meta:
        model = Cart
        fields = ['id', 'user', 'items']


class CartActionsSerializer(serializers.HyperlinkedModelSerializer):
    name = serializers.ReadOnlyField(source='product.name')
    price = serializers.ReadOnlyField(source='product.price')
    image = serializers.ReadOnlyField(source='product.image')  # Adding this line gives error

    class Meta:
        model = CartActions
        fields = ['name', 'price', 'image', 'quantity']

This is the error I'm getting when hitting the API:

UnicodeDecodeError at /store/api/cart/

'utf-8' codec can't decode byte 0xff in position 0: invalid start byte

This is the sample response I'm getting from /api/products API:

{
    "id": 1,
    "name": "Product 1",
    "description": "This is a sample description",
    "price": 500,
    "image": "http://192.168.43.210:9000/media/sample_product.jpeg"
}

I've tried almost all references in stackoverflow and other websites but I cannot find the exact same issue anywhere.

Answer 1

In this serializer you did not define field serializers explicitly, only mentioned field list:

class ProductSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Product
        fields = ['id', 'name', 'description', 'price', 'image']

DRF recognized image as an ImageField and applied serializers.ImageField serializer which has use_url argument:

use_url - If set to True then URL string values will be used for the output representation. If set to False then filename string values will be used for the output representation. Defaults to the value of the UPLOADED_FILES_USE_URL settings key, which is True unless set otherwise.

Thus it was serialized with ProductSerializer as an URL rather then binary picture or a filename.

In the other serializer you have defined field serializer explicitly:

class CartActionsSerializer(serializers.HyperlinkedModelSerializer):
    ...
    image = serializers.ReadOnlyField(source='product.image')

ReadOnlyField is a generic serializer which outputs source as it is . Thus 'product.image' returns an ImageField instance which is not an URL or path - it's the whole object with many attributes.

So try using a specific field serializer

serializers.ImageField(read_only=True, ...)

or make source more specific with default to avoid errors if image is missing

serializers.ReadonlyField(source='product.image.url', default=None, ...)

or access this value via method

class CartActionsSerializer(serializers.HyperlinkedModelSerializer):
    ...
    image = serializers.SerializerMethodField()

    def get_image(self, obj):
        return obj.product.image.url