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Copy fields from one instance to another in Django

I have the following code that takes an existing instance and copies or “archives” it in another model, and then deletes it, replacing it with a draft.

Current code

def archive_calc(self, rev_num, primary_field):
    model_a = Calc.objects.get(tag_number__tag_number = primary_field, revision_number = rev_num) #Current Revision instance
    model_b = CalcArchive() #Draft instance

    #Copies data to archive model
    for field in model_a._meta.fields:
        setattr(model_b, field.name, getattr(model_a, field.name))

    model_b.pk = None
    model_b.current_revision = False
    model_b.save()

    model_a.delete()

This works fine, but I need to change the system so that some models have foreign keys, since when archiving / deleting an instance, related records are deleted along with it. Therefore, my idea to fix this is to make changes to the draft records copied to the previous record, and then delete the project, saving the records associated with the foreign key.

Solution idea

def archive_calc(self, rev_num, primary_field):
    model_a = Calc.objects.get(tag_number__tag_number = primary_field, revision_number = rev_num) #Current Revision instance
    model_b = CalcArchive() #Archive Instance
    model_c = Calc.objects.get(pk = self.object.pk) #Draft instance

    #Copies data to archive model
    for field in model_a._meta.fields:
        setattr(model_b, field.name, getattr(model_a, field.name))

    model_b.pk = None
    model_b.current_revision = False
    model_b.save()

    #Copies data from draft instance to current revision instance
    for field in model_c._meta.fields:
        setattr(model_a, field.name, getattr(model_c, field.name))

    model_c.delete()

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def archive_calc(self, rev_num, primary_field):
    model_a = Calc.objects.get(calc_details__calc_serial_number = primary_field, revision_number = rev_num)
    model_b = CalcArchive()

    object_list_annual = model_a.calcreview_set.filter(calc__calc_details = primary_field)
    object_list_ageing = model_a.calcitem_set.filter(calc__calc_details = primary_field)

    for obj in object_list_annual:
        obj.calc_id = self.object.id
        obj.save()
    for obj in object_list_ageing:
        obj.calc_id = self.object.id
        obj.save()

    for field in model_a._meta.fields:
        setattr(model_b, field.name, getattr(model_a, field.name))
    model_b.pk = None
    model_b.current_revision = False
    model_b.save()

    model_a.delete()

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obj = Model.objects.get(pk=1)
obj.pk = get_unused_pk()
obj.save()

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