Re: How to hash fields and detect changes in a record

On Sat, Jun 11, 2022 at 12:13:16AM +1000, Mike Dewhirst wrote:
> On 10/06/2022 11:24 pm, Ryan Nowakowski wrote:
> > On Fri, Jun 10, 2022 at 05:52:48PM +1000, Mike Dewhirst wrote:
> > > 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.
>
> These are automatically generated notes which taken together constitute
> advice on how to deal with the analysis. Users can edit them. For example,
> someone might record some action taken regarding the advice. I don't want to
> delete that. If nothing has been edited, it is safe to delete.
>
> So how do I know it is the same as when originally generated - and safe to
> delete - except by storing a hash of the interesting fields.

Because when the user edits a note, during the form.save()(assuming
you're using Django forms), you'll set `altered_by_user` to True.

> And if that is the best approach, what sort of hashing will survive Python
> upgrades etc?

Pick a hash algorithm[1](ex: sha256). The output will remain the same
even with Python upgrades.

[1] https://docs.python.org/3/library/hashlib.html

> > 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.
>
> Not sure this would work. Note creation and eventually automatic deletion is
> all driven from model methods executed on saving.

Why wouldn't this work? During note creation, altered_by_user would be
set to False automatically because that's the default. When
automatically deleting, do:

Note.objects.filter(altered_by_user=False).delete()

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