Partial updates always change fields that are not in data, read only, and have a default

[melinath]

This is kind of an edge case, maybe? Maybe not. I ran into it because I have a field which is readonly on update but has a default for the create case.

See Serializer._writable_fields. I'm trying to do a partial update on one field, but I have another field that's readonly on update. BUT instead of being left alone, since it has a default, it always gets set TO THAT DEFAULT.

From my reading of writable_fields, it looks like this would happen with any field that has a default and isn't included in a partial update, but for it to happen on a read_only field seems particularly egregious to me. :-p

[melinath]

(My expectation would be that a partial update would only affect the fields that have data being sent their way.)

[tomchristie]

Is this still occurring against the latest release?

[melinath]

I tried upgrading to the latest stable release (3.3.1) and still had the issue, and I didn't see any changes between that and master in the relevant parts of the code.

[gcbirzan]

This is still present. The documentation (3.0 announcement, which is the only place I could find a reference to this) is also wrong: it doesn't mention that it'll only be used for read_only fields.

[tomchristie]

Milestoning to confirm against latest version.

[tomchristie]

Right - thanks for having raised this @melinath, and for having confirmed @gcbirzan.
Fix now incoming.

(Yay for now having a funded project where long-standing issues like this can finally get resolved!)

[MarkusH]

I just updated to a DRF 3.4.5 and noticed this change in behavior between 3.4.1 and 3.4.2. This change is breaking a couple of tests for us and is also changing the semantics of how defaults are treated in DRF.

May I ask to not have such backwards incompatible changes in patch releases in future versions. Thank you.

[tomchristie]

Sure. That's always a bit more gray when the behavior is classified as a bug, but yes I do think that we'll start being more cautious about any change that affects behavior, even in bug cases, and deferring those to major releases.

[MarkusH]

Thanks @tomchristie 😄