efficient model history using database triggers
Project description
Django Chronicle is an implementation of the slowly changing dimensions type 4 which uses database triggers.
How to use?
-
Create a custom revision model. e.g.
from chronicle.models import AbstractRevision class Revision(AbstractRevision): user = models.ForeignKey(settings.AUTH_USER_MODEL) created = models.DateTimeField(auto_now_add=True)
-
Set
settings.REVISION_MODEL
to point to your revision model. e.g.REVISION_MODEL = 'revision.Revision'
-
Let your models inherit from
HistoryMixin
e.g.from chronicle.models import HistoryMixin from django.db import models class Food(HistoryMixin, models.Model): name = models.CharField(max_length=50)
-
Create all the migrations and run them:
$ manage.py makemigrations $ manage.py migrate
That should create all the
_history
tables for your models that inherit from theHistoryMixin
. -
Create the database triggers
$ manage.py create_history_triggers
Now every change to your models should be logged in the _history
tables
and you can access the model history via the History
model which becomes
a field of the original class.
Example usage:
# create
food = Food('Carot')
food.save()
assert(Food.History.objects.filter(id=food.id).count() == 1)
# update
food.name = 'Carrot'
food.save()
assert(Food.History.objects.filter(id=food.id).count() == 2)
# delete
food.delete()
assert(Food.History.objects.filter(id=food.id).count() == 3)
Why database triggers?
The obvious choice to implement model history would be to connect a signal
handler to the post_save
and post_delete
signal. This has some rather huge
downsides:
1.) QuerySet.update()
and a lot of other QuerySet
methods do not emit any
signals. Having to limit the code to only use save()
can be a rather huge
performance problem depending on the type of application.
2.) There is a rather large performance impact when creating the history via
the Django ORM. A single QuerySet.update()
call could result in hundreds
or thousands of inserts. While this could mostly be solved using the
Manager.bulk_create
method a database trigger is a lot faster as there is no
extra database roundtrip required.
3.) This works for any kind of raw query - even outside of the Django ORM - as
long as the chronicle.revision_id
session variable is properly set.
The only real downside is the DB compatibility. Right now this package only supports the PostgreSQL database engine.
How to issue queries without the Django ORM?
Create a revision by inserting a row into the revision table and set the
chronicle.revision_id
session variable like so:
SET chronicle.revision_id = 42; -- replace 42 by the actual revision id
Once you have made all changes to your models don't forget to reset the session variable. Otherwise you might reuse the same revision by accident in the same DB session:
SET chronicle.revision_id TO DEFAULT;
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