On Fri, Jun 10, 2022 at 05:52:48PM +1000, Mike Dewhirst wrote:
> The use case is auto-deletion of out-of-date records if they have not
> changed.
>
> That might sound weird but it is the solution I have come to for a
> particular problem. My software analyses chemical properties and writes note
> records containing advice, each with a FK to the chemical.
>
> When values change sufficiently on the chemical, the software would
> construct a set of mostly different note records. The problem is that note
> records still exist from the previous set of properties. These would
> definitely confuse the user and thereby invalidate the advice.
You might consider versioning your chemical model objects. Then when
values change sufficiently on the chemical model object, you can create
a new version of the chemical object, leaving the old notes associated
with the old version of the chemical object. In your web app, you could
allow the users to browse old versions of the chemical including the
notes which may have been altered.
> The workaround is for the user to delete all notes *prior* to re-saving and
> auto-generating a new correct set of notes. There is a proviso that you
> wouldn't want to delete notes altered by users. I would document that so
> users understand why the software skipped deleting those notes.
>
> I think the solution might be to hash note.title and note.note into a new
> field note.hash on being auto-created. On subsequent saves, compare the
> latest hash with note.hash to decide whether to delete auto-inserted notes
> prior to generating the next set. Those subsequent saves could be months or
> years later.
Hashing is useful if you want to check that something has been
unexpectedly changed. I assume the note can only be changed through
your web app so you know when a user is changing a note. Since you're
expecting users to change some of the notes and you know when they do,
hashing might be overkill. Instead, add a boolean `altered_by_user`
field to the note model. Initially when you automatically create the
note altered_by_user would be set to False. If a user changes the note,
set altered_by_user to True.
> If unchanged, the old note is safe to delete because it is no longer
> relevant.
>
> I've googled around and there are lots of possible solutions but it seems
> the major problem might be that hashes are difficult to guarantee when the
> environment - such as the version of Python - changes.
>
> Also, I'm not convinced I have chosen the correct strategy.
>
> Hope I've explained the problem adequately.
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