Django maintain versions of a model object

Question

I have a model Goal which has two M2M fields and is referred to by another model Event. Now, the requirement is such that Goal is editable which means I can add/delete from M2M fields and add/delete events. But editing goes through moderation and only after approval the changes are to be reflected or in case of rejection changes must be reverted.

I have gone through deepcopy but it doesn't serve my purpose because lets say if I do x=deepcopy(goal object) I get copy of goal object but I am unsure what I can do with it.

Then I came to know about model_to_dict from django.forms module it serializes the whole object which is handy. I was thinking may be I could use this dictionary to create a model RevisedGoal with original goal as foreign key. Is it a feasible solution? Are there any othe ways of achieving this?

Answer 1

I think that you're essentially asking how you can keep track of changes to Goal's M2M fields such that they can be approved or reverted. If you need to persist these pending changes to the database to approve or deny later, then cloning the model may not be the best solution, particularly if you want to revert. If you expect a lot of these changes, you'll want to separate the potential changes from the "live" instances of your Goals and Events.

I would suggest creating a new model that is linked to Goal and represents these pending changes. This object would keep track of added/removed records from the Goals M2M fields, and could contain some additional fields related to approval (like who approved the changes and when). When one of these new objects gets marked approved, you can make the requisite changes on the original Goal instance.

How you keep track of the added/removed M2M fields is the tricky part. In the example below, I've just created corresponding M2Ms on the pending changes object that you could iterate through on approval to apply to the original Goal object.

class Goal(models.Model):
...


class PendingGoalChange(models.Model):
    goal = models.ForeignKey(Goal, related_name='changes')
    approved = ...
    approver = ....

    added_m2m_field_instances = models.ManyToMany(...)
    removed_m2m_field_instances = models.ManyToMany(...)

    def approve(self):
        self.approved = True
        ...
        for new_field in self.added_m2m_field_instances.all():
            self.goal.field.add(new_field)