DRF Error: Cannot assign must be a instance

Question

I'm working on an app that lets you track your expenses and I'm trying to create an 'balnace' object when the user registers, but when I try so I get an Error: Cannot assign "15": "Balance.user_id" must be a "Users" instance.

Model

class Balance(models.Model):

balance = models.FloatField(
    verbose_name="Balance", blank=False, null=False)
user_id = models.ForeignKey(
    'expenseApi.Users', verbose_name="Usuario", blank=False, null=False, on_delete=models.PROTECT)

def __str__(self):
    return '{}'.format(self.pk)

serializer

class RegisterSerializer(serializers.ModelSerializer):

tokens = serializers.SerializerMethodField()
email = serializers.EmailField(max_length=50)
class Meta:
    
    model = Users
    fields= ['id', 'email', 'password', 'name', 'lastname', 'birthday', 'tokens']
    
    extra_kwargs = {
        'password': {
            'write_only': True,
        },
    }

def get_tokens(self,user):
        refresh = RefreshToken.for_user(user)

        data = {
            'refresh': str(refresh),
            'access': str(refresh.access_token),
        }
        return data    

def create(self, request):

    email= Users.objects.filter(email=request['email'])

    if email:
        raise serializers.ValidationError({'detail': 'El email ya esta en uso'})

    user = Users.objects.create_user(email=request['email'],
                                    password=request['password'],
                                    name=request['name'],
                                    lastname=request['lastname'],
                                    birthday=request['birthday'])

    user['balance'] = Balance.objects.create(balance=0,user_id=user.id)

    return user

Answer 1

I didn't go in detail about logic on your code but I guess error based on your code at

 user['balance'] = Balance.objects.create(balance=0,user_id=user.id)

should be

 user['balance'] = Balance.objects.create(balance=0, user_id=user)

Answer 2

Since you named your ForeignKey user_id, this means that you can assign a User object to .user_id, or the primary key to .user_id_id:

user['balance'] = Balance.objects.create(
    balance=0,
    user_id_id = user.id
)

I would however advise to rename user_id to user, since now the field "hints" that it is an id, but it is not.

class Balance(models.Model):
    balance = models.FloatField(verbose_name='Balance')
    user = models.ForeignKey(
        'expenseApi.Users',
        verbose_name='Usuario',
        on_delete=models.PROTECT
    )
    # …

Then we thus can set this with:

user['balance'] = Balance.objects.create(
    balance=0,
    user_id = user.id
)