archival patterns/mixins?

[Tim Chase]

I've been trying to reduce some code in a new project, and I have a
number of objects where I have the main class, and an archival
version of the same class, something like

def Thing(Model):
last_updated = DateTimeField(...)
who_updated = ForeignKey(User)
field1 = ...
field2 = ...
...

def ThingArchive(Model):
# pseudo-type declaration
original_id = Thing.id.__class__ # whether Int or Guid

last_updated = DateTimeField(...)
who_updated = ForeignKey(User)
field1 = ...
field2 = ...
...

such that every time Thing is updated+saved, the previous copy's
information gets archived in ThingArchive, storing the original
Thing.id as original_id (the id might be an integer or a GUID
depending on the table).

For one class, this isn't a particularly big deal, but I've got a
number of these class-pairs and the code duplication is starting to
get out of hand. Is there a pattern I should be using to abstract
this archiving logic?

Thanks,

-tkc

[Bill Freeman]

Use one model with a field to indicate whether or not this is the archival (backup?) version.  This could be your original_id field, which might be better represented as a null-able foreign key field on self.  Update step:

    delete any instances whose "original" is this field (the reverse reference manager works for this)

    save a copy of the current instance (with any changes you want, and a null "original" field)

    update the "original" field of the old copy to point to the new one.

This can be extended to more than one level of backup.

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