I would say that the changeset itself does not know your intent, will this become an insert or update or just a form validation, so it cannot return an error.
For the call to update, I think 2 things matter, first if the value hasn’t changed ecto will never attempt to write to the database, and secondly if that value shouldn’t be written to the database and no other fields have changed what then should be in the action to the database.
I have never used :writable, so I may be completely of the mark.
