python 50 lines · 8 steps

Tracking field changes in a Django model

An Order model snapshots audited fields on load so save() can log exactly what changed.

Explained by highlit
Django Intermediate change-tracking audit-log model-lifecycle snapshot dirty-fields
1from django.db import models
2 
3 
4class Order(models.Model):
5 STATUS_CHOICES = [
6 ("pending", "Pending"),
7 ("paid", "Paid"),
8 ("shipped", "Shipped"),
9 ("cancelled", "Cancelled"),
10 ]
11 
12 reference = models.CharField(max_length=32, unique=True)
13 status = models.CharField(max_length=16, choices=STATUS_CHOICES, default="pending")
14 total = models.DecimalField(max_digits=10, decimal_places=2)
15 
16 AUDITED_FIELDS = ("status", "total")
17 
18 def __init__(self, *args, **kwargs):
19 super().__init__(*args, **kwargs)
20 self._snapshot = self._current_state()
21 
22 def _current_state(self):
23 return {field: getattr(self, field) for field in self.AUDITED_FIELDS}
24 
25 def changed_fields(self):
26 current = self._current_state()
27 return {
28 field: (self._snapshot[field], current[field])
29 for field in self.AUDITED_FIELDS
30 if self._snapshot[field] != current[field]
31 }
32 
33 def save(self, *args, **kwargs):
34 is_new = self._state.adding
35 changes = self.changed_fields()
36 
37 super().save(*args, **kwargs)
38 
39 if not is_new and changes:
40 AuditLog.objects.bulk_create([
41 AuditLog(
42 content_object=self,
43 field=field,
44 old_value=str(old),
45 new_value=str(new),
46 )
47 for field, (old, new) in changes.items()
48 ])
49 
50 self._snapshot = self._current_state()
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Snapshotting values at load time lets you diff against the current state later without extra queries.
  2. 2Overriding save() around super().save() is the natural hook for before/after side effects.
  3. 3Comparing the snapshot to current state only after a successful save keeps your audit log honest.

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