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

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. There's a bit mentioned about it in the serializer documentation.

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

I believe 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 

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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