How to validate a field on update in DRF

Question

I have a serializer for a model with a foreign key. The requirement is that on create, foreign key can be set to any existing object from the related model, but on update the related object cannot be changed. I can check this in the custom update(), but it would be more elegant to use serializer validation to check for this? But I am not sure how. Example code:

class Person(models.Model):
    name = models.CharField(max_length=256)
    spouse = models.ForeignKey(Person)

class PersonSerializer(serializers.ModelSerializer):
    class Meta:
        model = Person

    # this is how I know how to do this
    def create(self, validated_data):
        try:
            spouse = Person.objects.get(pk=int(validated_data.pop('spouse')))
        except Person.DoesNotExist:
            raise ValidationError('Imaginary spouses not allowed!')
        return Person.objects.create(spouse=spouse, **validation_data)

    def update(self, person, validated_data):
        if person.spouse.pk != int(validated_data['spouse']):
            raise ValidationError('Till death do us part!')
        person.name = validation_data.get('name', person.name)
        person.save()
        return person

   # the way I want to do this
   def validate_spouse(self, value):
       # do validation magic

Answer 1

Hello @kartik,

You can definitely do this using the validation on a field. The way you'd check if it's an update vs. creation is checking for self.instance in the validation function. 

self.instance will hold the existing object and it's values, so you can then use it to compare against.

 This should work for your purposes:

def validate_spouse(self, value):
    if self.instance and value != self.instance.spouse:
        raise serializers.ValidationError("Till death do us part!")
    return value

Another way to do this is to override if the field is read_only if you're updating. This can be done in the __init__ of the serializer. Similar to the validator, you'd simply look for an instance and if there's data:

def __init__(self, *args, **kwargs):
    # Check if we're updating.
    updating = "instance" in kwargs and "data" in kwargs

    # Make sure the original initialization is done first.
    super().__init__(*args, **kwargs)

    # If we're updating, make the spouse field read only.
    if updating:
        self.fields['spouse'].read_only = True

Hope it helps!!
Thank You!!