Django Model Field Default Based Off Another Field in Same Model

Question

I have a model that I would like to contain a subjects name and their initials (he data is somewhat anonymized and tracked by initials).

Right now, I wrote

<         


        

Answer 1

Using Django signals, this can be done quite early, by receiving the post_init signal from the model.

from django.db import models
import django.dispatch

class LoremIpsum(models.Model):
    name = models.CharField(
        "Name",
        max_length=30,
    )
    subject_initials = models.CharField(
        "Subject Initials",
        max_length=5,
    )

@django.dispatch.receiver(models.signals.post_init, sender=LoremIpsum)
def set_default_loremipsum_initials(sender, instance, *args, **kwargs):
    """
    Set the default value for `subject_initials` on the `instance`.

    :param sender: The `LoremIpsum` class that sent the signal.
    :param instance: The `LoremIpsum` instance that is being
        initialised.
    :return: None.
    """
    if not instance.subject_initials:
        instance.subject_initials = "".join(map(
                (lambda x: x[0] if x else ""),
                instance.name.split(" ")))

The post_init signal is sent by the class once it has done initialisation on the instance. This way, the instance gets a value for name before testing whether its non-nullable fields are set.