[Django]-How to validate a field on update in DRF?

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

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