PUT request to Django API to update list of nested objects with Many To Many relationships

Question

I have a user model with a ManyToMany relationship with another Model

class User(AbstractUser):
    countries = models.ManyToManyField(Country, blank=True)
    count = models.IntegerField(blank=True, default=0)

    def save(self, *args, **kwargs):
        # Must save model before Many To Many relationship can be used.
        super(User, self).save(*args, **kwargs)
        self.count = self.countries.count()
        super(User, self).save(*args, **kwargs)

and my country model is a rather large model:

class Country(models.Model):
    '''
    Describes the countries, as well as territories of the world.
    '''
    name = models.CharField(max_length=255, null=True, blank=True)
    top_level_domain = JSONField(null=True, blank=True)
    alpha2code = models.CharField(max_length=255, null=True, blank=True)
    alpha3code = models.CharField(max_length=255, null=True, blank=True)
    calling_codes = JSONField(null=True, blank=True)
    capital = models.CharField(max_length=255, null=True, blank=True)
    alt_spellings = JSONField(null=True, blank=True)
    region = models.CharField(max_length=255, null=True, blank=True)
    subregion = models.CharField(max_length=255, null=True, blank=True)
    population = models.IntegerField(null=True, blank=True)
    latlng = JSONField(null=True, blank=True)
    demonym = models.CharField(max_length=255, null=True, blank=True)
    area = models.FloatField(null=True, blank=True)
    gini = models.FloatField(null=True, blank=True)
    timezones = JSONField(null=True, blank=True)
    borders = JSONField(null=True, blank=True)
    native_name = models.CharField(max_length=255, null=True, blank=True)
    numeric_code= models.CharField(max_length=255, null=True, blank=True)
    currencies = models.ManyToManyField(Currency)
    languages = models.ManyToManyField(Language)
    flag = models.CharField(max_length=255, null=True, blank=True)
    regional_blocs = models.ManyToManyField(RegionalBloc, blank=True)
    cioc = models.CharField(max_length=255, null=True, blank=True)

    def __str__(self):
        return self.name

I'm trying to make a put request from my frontend to update the list of countries associated with (data is sent as an array of objects from React with axios). I got the error "AssertionError: The .update() method does not support writable nested fields by default. Write an explicit .update() method for serializer api.serializers.UserDetailSerializer, or set read_only=True on nested serializer fields." Since I obviously can't make my the countries read_only=True, I'm trying to add an update method to my serializer.

class UserDetailSerializer(UserDetailsSerializer):
    countries = CountrySerializer(many=True)
    count = serializers.IntegerField(read_only=True)

    class Meta:
        model = User
        fields = ('pk', 'username', 'email', 'count', 'countries')

    # Update the instance upon Put Request from frontend.
    def update(self, instance, validated_data):
        instance.countries = validated_data['countries']
        instance.save()
        return instance

This update method gives me the error 'Direct assignment to the forward side of a many-to-many set is prohibited. Use countries.set() instead.'

When I change the update method to

  instance.countries.set(validated_data['countries'])

I get the error 'TypeError: unhashable type: 'collections.OrderedDict''. Starting to feel a bit lost, and like I'm running in circles. How do I write the update method correctly?

edit: What I'm trying to do is just add/remove a Country object to the Users country list. I don't actually want to edit any of the country objects.

my country data being posted would be a list of countries objects looking like this

{
    "id": 6,
    "currencies": [
        {
            "code": "EUR",
            "name": "European Euro",
            "symbol": "€"
        }
    ],
    "languages": [
        {
            "iso639_1": "ca",
            "name": "Catalan",
            "native_name": "Català"
        }
    ],
    "regional_blocs": [],
    "name": "Andorra",
    "top_level_domain": [
        ".ad"
    ],
    "alpha2code": "AD",
    "alpha3code": "AND",
    "calling_codes": [
        "376"
    ],
    "capital": "Andorra la Vella",
    "alt_spellings": [
        "AD",
        "Principality of Andorra",
        "Principat d'Andorra"
    ],
    "region": "Europe",
    "subregion": "Southern Europe",
    "population": 78014,
    "latlng": [
        42.5,
        1.5
    ],
    "demonym": "Andorran",
    "area": 468.0,
    "gini": null,
    "timezones": [
        "UTC+01:00"
    ],
    "borders": [
        "FRA",
        "ESP"
    ],
    "native_name": "Andorra",
    "numeric_code": "020",
    "flag": "https://restcountries.eu/data/and.svg",
    "cioc": "AND"
}

nebuler · Accepted Answer · 2018-11-09 21:20:15Z

1

You get TypeError: unhashable type: 'collections.OrderedDict' because you are trying to assign an object of type collections.OrderedDict to countries, which is a ManyToManyField.

Do the following instead:

def update(self, instance, validated_data):
    country_names = [cdata['name'] for cdata in validated_data['countries']]
    countries = Country.objects.filter(name__in=country_names)
    instance.countries.set(countries)
    return instance

answered Nov 9, 2018 at 21:20

nebuler

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2 Comments

peter Over a year ago

Thanks, that was very easy. .set() has to be a queryset, of the appropriate model?

nebuler Over a year ago

@peter176 It doesn't have to be a queryset, it could be any iterable (list, set, etc). And yes, you want the elements to be of the appropriate model.

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