Keeping history of Django model (by using memoization?)

Question

I'm working on a Django project for which I'll need to store the historic information of many models.

So far, I'm using the following strategy:

1. Memoize the relevant values in __init__.
2. When saving, check if a relevant value has changed.
3. If a relevant value has changed, save memoized values into a "history" model.

My code goes as follows:

from django.db import models
from core.models import User

class Spam(models.Model):
    user= models.OneToOneField(User, on_delete=models.PROTECT, related_name='spam')
    ham = models.CharField(max_length=10)
    eggs = models.IntegerField()

    def __init__(self, *args, **kwargs):
        super(Spam, self).__init__(*args, **kwargs)
        self.__relevant_fields = ['ham', 'eggs']
        self._memoize()

   def _memoize(self):
       for f in self.__relevant_fields:
           setattr(self, '__prev_%s' % f, getattr(self, f))

   def _get_prev_value(self, f):
       return getattr(self, '__prev_%s' % f)

   def _dirty_field(self, f) -> bool:
       return getattr(self, '__prev_%s' % f) != getattr(self, f)

   def _dirty(self) -> bool:
       return any([self._dirty_field(f) for f in self.__relevant_fields])

   def _save_history(self):
       history = dict()
       for f in self.__relevant_fields:
           history[f] = self._get_prev_value(f)
       self.user.spam_history.create(**history)

   def save(self, *args, **kwargs):
       super(Spam, self).save(*args, **kwargs)
       if self._dirty():
           self._save_history()

class SpamHistory(models.Model):
    user = models.ForeignKey(User, on_delete=models.PROTECT, related_name='spam_history')
    ham = models.CharField(max_length=10)
    eggs = models.IntegerField()

Although this approach works, there are some things that make me uncomfortable:

1. I need to do this for many models, so I'm repeating myself a lot! I'm looking into inheritance, mixins or the like, but so far I haven't found a right way to make it work.
2. Is this the best approach to achieve what I want? Considering that I will need to fetch historic records frequently, I need the "history" model for each model I need to keep history from.

So, my specific questions are:

• Am I implementing a good strategy for keeping the history of my model(s)?
• Is there a way to improve this strategy by using an inheritance or mixin strategy?

Answer 1

Ok, after googling a bit more, I found a possible solution: django-simple-history:

from django.db import models
from core.models import User
from simple_history.models import HistoricalRecords

class Spam(models.Model):
    user = models.OneToOneField(User, on_delete=models.PROTECT, related_name='spam')
    ham = models.CharField(max_length=10)
    eggs = models.IntegerField()
    history = HistoricalRecords()

This seems to do exactly what I need to do. I'll test it.