Django Model Admin Fields

Django is one of the most popular Python web frameworks today. Importantly it provides an ORM permitting us to define models as Python classes that Django maps to a databse representations for us.

A very common, nearly ubiquitous desire/need I have is to keep some record against all database objects as to who created them, when and who last edited them and when. Very basic tracking fields. Given the ubiquity of the need it was best implemented as an abstrack model that my models derive from.

The basic Django example of model:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

can be extended thusly:

from django.db import models
from django_model_admin_fields import AdminModel

class Person(AdminModel):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

which has the simple effect of adding the following fields to the model silently:

    created_by = models.ForeignKey(User, verbose_name='Created By', 
                                   related_name='%(class)ss_created', 
                                   editable=False, null=True, on_delete=models.SET_NULL)
    created_on = models.DateTimeField('Time of Creation', editable=False, null=True)
    created_on_tz = TimeZoneField('Time of Creation, Timezone', 
                                  default=settings.TIME_ZONE, editable=False)

    last_edited_by = models.ForeignKey(User, verbose_name='Last Edited By', 
                                       related_name='%(class)ss_last_edited', 
                                       editable=False, null=True, on_delete=models.SET_NULL)
    last_edited_on = models.DateTimeField('Time of Last Edit', editable=False, null=True)
    last_edited_on_tz = TimeZoneField('Time of Last Edit, Timezone', 
                                      default=settings.TIME_ZONE, editable=False)

(a more precise description of course is in __init__.py)

Importantly it also overrides the mmodel save() method to set those six fields before calling super().save() (i.e. the default save method) and thus these fields are automatically managed.

The currently active Django timezone is saved as well to support sensible human interpretation of the saved times (as Django's DateTimeField) is not timezone aware.

To make use of that easier two properties are also added to the model: created_on_local and last_edited_on_local which are timezone aware versions of the naive created_one and last_edited_on fields.

To illustrate use of the Person example above:

person = Person()
person.first_name = "John"
person.last_name = "Smith"
person.save

print(f"{person.first_name} {person.last_name}")
print(f"was created by {person.created_by} on {person.created_on_local}.")

Of course to make use of local times, you need to activate the timezone that the creating user is in. To that you need to know it first. The Javascript library jstz is useful in that regard for detecting the users timezone and there's a great guide on setting timezones in Django inthe django documentation proper.