Add changeable option for fields.

[calvin620707]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Purpose

This is a feature request to add a new option, changeable, for fields to decide whether the field is changeable during put/patch.

Proposal

import requests
from rest_framework import serializers

class FooSerializer(serializers.Serializer):
    unchangeable = serializers.CharField(changeable=False)

Expected behavior

resp = requests.post('/foo', json={'unchangeable': 'abc'})
assert resp.status_code == 200

foo_id = resp.json()['id']

resp = requests.patch(f"/foo/{foo_id}", json={'unchangeable': 'changed!'})
assert resp.status_code == 400
assert resp.json() == {'errors': {'unchangeable': 'This field is unchangeable'}}

resp = requests.patch(f'/foo/{foo_id}', json={'unchangeable': 'abc'})
assert resp.status_code == 200

# expected the same behavior for put.

Pseudo code

class Field(object):
    def __init__(self, ..., changeable=True):
        self.changeable = changeable
        # ...
    
    def validate_changeable(self, data):
        if self.changeable or self.parent is None or self.parent.instance is None:
            return
        
        original_data = getattr(self.parent.instance, self.field_name)
        if self.compare:
            is_changed = self.compare(original_data, data) != 0
        else:
            is_changed = original_data != data
        
        if is_changed:
            raise ValidationError("This field is unchangeable.")

    def run_valdations(self, data=empty):
        self.validate_changeable(data)
        ...

I can help to send the MR for this feature if the feature is acceptable.

[rpkilby]

Hi @calvin620707. I don't think this is something we'd want to merge into the core framework, as it seems like a fairly niche case with no single correct implementation. e.g., another user might have a different expectation in that an unchangeable field should effectively become read-only for updates, and that including the field in the request body should be invalid.

My recommendation would be to implement this as a validator class, or as a utility function that can be called from the validate_<field> hook. Note that the validator API will change with #6172.

[calvin620707]

validator class seems like a reusable way. Thanks for pointing that out.