Creating and saving foreign key objects using a SlugRelatedField - django

Question

I've just started with Django REST framework and I'm having trouble with saving foreign keys. I have a Merchant model and a Phone model. The Phone has a foreign key to Merchant. When making a POST request to Merchant, I want to create Phone objects for the numbers provided in the request. But when I supply the phone numbers, it gives me the following error
Object with phone=0123456789 does not exist.
I just want it to create the Phone object itself. Here are the models that I am using:
class Merchant(models.Model):
merchant_id = models.CharField(max_length=255)
name = models.CharField(max_length=255)
is_active = models.BooleanField(default=True)
class Meta:
managed = True
db_table = 'merchant'
# Managers
objects = models.Manager()
active = managers.ActiveManager()
class Phone(models.Model):
phone = models.CharField(max_length=255)
merchant = models.ForeignKey('merchant.Merchant',
related_name='phones',
blank=True,
null=True)
class Meta:
managed = True
db_table = 'phone'
And here is the view and serializer that I am using them with
class MerchantSerializer(serializers.ModelSerializer):
phones = serializers.SlugRelatedField(
many=True,
slug_field='phone',
queryset=primitives.Phone.objects.all())
class Meta:
model = Merchant
fields = (
'merchant_id',
'name',
'is_active',
'phones',
)
class MerchantViewSet(viewsets.ModelViewSet):
queryset = Merchant.active.all()
serializer_class = MerchantSerializer
Here's what my request body looks like:
{
"merchant_id": "emp011",
"name": "Abhinav",
"is_active": true,
"phones": [
"0123456789",
"9876543210"
]
}
Here's the response:
400 Bad Request
{"phones":["Object with phone=0123456789 does not exist."]}

Answer 1

The SlugRelatedField provided by Django REST framework, like many of the related fields, is designed to be used with objects that already exist. Since you are looking to reference objects which already exist, or object which need to be created, you aren't going to be able to use it as-is.
You will need a custom SlugRelatedField that creates the new object when one doesn't exist.
class CreatableSlugRelatedField(serializers.SlugRelatedField):
def to_internal_value(self, data):
try:
return self.get_queryset().get_or_create(**{self.slug_field: data})[0]
except ObjectDoesNotExist:
self.fail('does_not_exist', slug_name=self.slug_field, value=smart_text(data))
except (TypeError, ValueError):
self.fail('invalid')
class MerchantSerializer(serializers.ModelSerializer):
phones = CreatableSlugRelatedField(
many=True,
slug_field='phone',
queryset=primitives.Phone.objects.all()
)
class Meta:
model = Merchant
fields = (
'merchant_id',
'name',
'is_active',
'phones',
)
By switching to get_or_create, the phone number object will be created if one doesn't already exist. You may need to tweak this if there are additional fields that have to be created on the model.

Answer 2

Building on Kevin's answer, a little cleaner IMO due to not using get_or_create and [0]
class CreatableSlugRelatedField(serializers.SlugRelatedField):
def to_internal_value(self, data):
try:
return self.get_queryset().get(**{self.slug_field: data})
except ObjectDoesNotExist:
return self.get_queryset().create(**{self.slug_field: data}) # to create the object
except (TypeError, ValueError):
self.fail('invalid')
Note the only required field in the related object should be the slug field.

Answer 3

You have to specify a value for phone field of the object Phone. If you want create phone object without specifying value for the field phone then you have to enable null and blank fields.
phone = models.CharField(max_length=255,null=true,blank=true)
If you still experience problems, make sure the post data contains the required fields. You can use ipdb for this.