I have a similar problem where I want to maintain an audit trail for
"manual overrides" to inputs to our risk loop. I haven't implemented it
in Django yet, but the way I've done this in the past is similar to
Waylan: using a history version of the table. There are a few
differences, however. My table's will probably look more like this:
ParameterOverride
id
name
value
user
note
ParameterOverrideHist
id
name
value
user
note
time_from
time_thru
The "hist" table simply adds the time_from/thru fields to track the
date range that this value was valid. Whenever a new row is added to
the main table, it is also added to the hist table (the id is *not*
auto_increment on the hist table). Then time_from is set to the current
time and time_thru is set to null. When a row is edited, a new row is
inserted into the hist table with time_from set to now and time_thru
set to null, but the old row that had time_thru null is updated so its
time_thru is the same as the new row's time_from. Deleted rows simply
update the row in the hist table with time_thru = null to time_thru =
the current time.
Thus you can query the hist table on any give date/time to get the
active row at that moment: "where _date_ >= time_from and (_date_ <
time_thru or time_thru is null)". While these queries are a little
ugly, this design allows you to get the row at a given time more
effectively that only storing the "updated" date in it (you have to
find the row with max(date) where date >= _eval_date_)
In my C#/Sql Server life, I handled this update logic using triggers on
the main table. In Django I'd probably override save() and delete()
like Waylan suggested.
I've been noodling on how to do this more easily since I'll have need
this approach quite a few times in my applications. Maybe a decorator?
It would be way cool to just say:
@track_audits()
class ParameterOverride(models.Model):
name = models.CharField(...)
value = models.FloatField(...)
user = models.ForeignKey(...)
note = models.TextField(...)
And it creates the hist table, overrides the save/delete methods and
maybe adds a few convenience methods like get_audit_trail(),
get_on_date(...)
Anyway, food for thought...
-Dave
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