Login

Full Model History

Author:
willhardy
Posted:
December 7, 2008
Language:
Python
Version:
1.0
Score:
2 (after 2 ratings)

This was a wild experiment, which appears to work!

One model holds all the data, from every object version ever to exist. The other model simply points to the latest object from its gigantic brother. All fields can be accessed transparently from the little model version, so the user need not know what is going on. Coincidently, Django model inheritance does exactly the same thing, so to keep things insanely simple, that's what we'll use: 

class EmployeeHistory(FullHistory):
    name = models.CharField(max_length=100)

class Employee(EmployeeHistory):
    pass

That's it! Django admin can be used to administer the Employee and every version will be kept as its own EmployeeHistory object, these can of course all be browsed using the admin :-)

This is early days and just a proof of concept. I'd like to see how far I can go with this, handling ForeignKeys, ManyToManyFields, using custom managers and so on. It should all be straightforward, especially as the primary keys should be pretty static in the history objects...

updated 3 August 2009 for Django 1.1 and working date_updated fields 

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
# -*- coding: UTF-8 -*-

from django.db import models
from datetime import datetime

class FullHistory(models.Model):
    """ 
    Issues: Unique fields don't work (multiple versions of the same row may need to have the same value)
    """
    date_created     = models.DateTimeField(default=datetime.now, editable=False)
    date_updated     = models.DateTimeField(default=datetime.now, editable=False)

    class Meta:
        abstract = True
        ordering = ('date_updated',)

    def __init__(self, *args, **kwargs):
        # History classes must end in 'History', others must not.
        if self.__class__.__name__.endswith('History'):
            self._history_class = True
        else:
            self._history_class = False
        super(FullHistory, self).__init__(*args, **kwargs)


    def save(self, *args, **kwargs):
        if self._history_class:
            self._save_history_instance()
        else:
            self._save_non_history_instance()

        super(FullHistory, self).save(*args, **kwargs)

    def _save_history_instance(self):
        """ Save method for a History object.
        """
        # This updated instance must become a new object, 
        # no updating is possible for history classes
        self.id = None

    def _save_non_history_instance(self):
        """ Save method for a non-History object.
        """
        # Duplicate and reassign parent.
        for model, field in self._meta.parents.items():
            if getattr(self, '%s_id' % field.name) is not None:
                rel_obj = getattr(self, field.name)
                rel_obj.id = None
                rel_obj.save()
                setattr(self, '%s_id' % field.name, rel_obj.id)

        # Set the new update time on the non-archived version
        self.date_updated = datetime.now()

    def delete(self):
        # Only delete if this is not a "history" version
        if not self._history_class:
            self.save()
            super(FullHistory, self).delete()

More like this

  1. Template tag - list punctuation for a list of items by shapiromatron 10 months, 2 weeks ago
  2. JSONRequestMiddleware adds a .json() method to your HttpRequests by cdcarter 10 months, 3 weeks ago
  3. Serializer factory with Django Rest Framework by julio 1 year, 5 months ago
  4. Image compression before saving the new model / work with JPG, PNG by Schleidens 1 year, 6 months ago
  5. Help text hyperlinks by sa2812 1 year, 6 months ago

Comments

carljm (on December 8, 2008):

Brilliant. Way to get around all the hard stuff in the AuditTrail wiki page in one fell swoop.

#

willhardy (on December 10, 2008):

Thanks! I'll look at expanding on this and making it comprehensive. I've just realised what a cool snippet number I have. Excellent.

#

Killarny (on December 16, 2008):

I am also highly interested in this, as we've done something similar at work. Originally we intended to do the history at the DB level, but after designing the models to support that (a custom model that did nothing but provide read access to history entries) we decided to use Django for it.

Looking forward to seeing what you come up with as you expand it!

#

andybak (on June 26, 2009):

Take a look at django-reversion. That has admin integration and will work on third party apps just by changing a line in their admin.py

#

pjv (on July 23, 2009):

is this currently working for you against django 1.1 rc?

when i try to use it, i get the following error on save of an "Employee" instance:

/Users/paul/Documents/web_development/django_sites/test_project/history/models.pyc in save(self, *args, **kwargs)
     18 
     19             # Duplicate and reassign parent
---> 20             for model, field in self._meta.parents:
     21                 if getattr(self, '%s_id' % field.name) is not None:
     22                     rel_obj = getattr(self, field.name)

TypeError: 'ModelBase' object is not iterable

#

Please login first before commenting.