Serializer ForeignKey results in "Expected a dictionary ..." - django

Question

My Model:
class Font(ValidateVersionOnSaveMixin, models.Model):
id = models.UUIDField(primary_key=True, editable=True)
name = models.CharField(max_length=100, blank=False, null=False)
class Glyph(ValidateVersionOnSaveMixin, models.Model):
id = models.UUIDField(primary_key=True, editable=True)
unit = models.CharField(max_length=100, blank=False, null=False, unique=True)
font = models.ForeignKey(Font, on_delete=models.CASCADE)
I want to post the following JSON to add a Glyph to an already existing Font (having the fontId as ID) object.
{
fontId: "4a14a055-3c8a-43ba-aab3-221b4244ac73"
id: "40da7a83-a204-4319-9a04-b0a544bf4440"
unit: "aaa"
}
As there is a mismatch between the ForeignKey Field font and the JSON propertyfontId I am adding source='font' in my Serializer:
class FontSerializer(serializers.ModelSerializer):
class Meta:
model = Font
fields = ('id', 'name')
class GlyphSerializer(serializers.ModelSerializer):
fontId = FontSerializer(source='font')
class Meta:
model = Glyph
fields = ('id', 'unit', 'fontId' )
But the result is an BAD REQUEST Error:
{"fontId":{"non_field_errors":["Invalid data. Expected a dictionary, but got str."]}}
Update
The following Serializer gave me the result I was looking for.
class GlyphSerializer(serializers.ModelSerializer):
fontId = serializers.PrimaryKeyRelatedField(
queryset=Font.objects.all(),
required=True,
source='font',
write_only=False
)
class Meta:
model = Glyph
fields = ('id', 'unit', 'version', 'fontId')

Answer 1

You can user model_to_dict method as bellow:
from django.forms.models import model_to_dict
model_to_dict(obj)

Answer 2

You have defined fontId as being a serialized object (FontSerializer). But that serializer in turn is defined as having both an id and a name. Where as your json dictionary is posting only an id. You would have to change that to a dictionary that contains both an id and a name
{
fontId: {id: "4a14a055-3c8a-43ba-aab3-221b4244ac73",name: "some name" },
id: "40da7a83-a204-4319-9a04-b0a544bf4440"
unit: "aaa"
}

Answer 3

The reason you are getting this error is that during deserialization process, DRF calls .is_valid(raise_exception=True) before you can call serializer.save(validated_data). And non_field_errors lists any general validation errors during this process. In your GlyphSerializer, your FontSerializer is a nested serializer, which correlates to a Python dictionary. So it will raise an error like you encountered for any non-dictionary data types.
You could create a subclass of GlyphSerializer called GlyphCreateSerializer
class FontSerializer(serializers.ModelSerializer):
class Meta:
model = Font
fields = ('id', 'name')
class GlyphSerializer(serializers.ModelSerializer):
fontId = FontSerializer(source='font')
class Meta:
model = Glyph
fields = ('id', 'unit', 'fontId' )
class GlyphCreateSerializer(GlyphSerializer):
fontId = serializers.CharField()
And you can use GlyphCreateSerializer for the POST request on your Viewset.