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?

• FMc: Third-party tools might exist that will do this for you.
• Barranka: @FMc I'll search for them (if you know of one, could you share it?)
• Barranka: @Fmc I've found django-model-changes-py3; however, it seems that this just saves the current state plus the two previous states of the model instance... I'd like to keep the full history.
• Barranka: @FMc Ook! I found what I need: django-simple-history! Right now I'm feeling a bit dumb I didn't found this earlier.
• Barranka: @Graipher indeed. Correcting now

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.